Ultimate Onions & Oven-Bottom Burgers

The onions were the star of this one. I started with three enormous onions in a cast-iron pan, reduced them slowly with butter, balsamic vinegar, Henderson’s Relish and black pepper, and ended up with something I’d happily eat straight out of the pan. No added sugar needed. They went on pork burgers in proper Lancashire oven-bottom muffins, with Blackjack Vintage cheddar on the bottom, a slice of American cheese over each patty and a frankly excessive amount of onions. The onions were a genuine 10/10, but on my first attempt, 13 October 2026, I cooked the burgers entirely in the oven. They were juicy, but too wet on the outside and missing a proper crust. I’ve since made them again with a reverse sear — gentle oven cooking followed by a hot pan finish — and the burgers were much better. That’s the version I’d recommend now. The photographs below show the original oven-only cook, not the later reverse-seared batch.

SERVES 2 · PREP 10–15 min · COOK roughly 40–60 min (mostly reducing the onions; actual burger time depends on thickness). The onion sauce quantities below are practical amounts for repeating the cook, not weights I recorded on the night.

Ingredients

• 3 very large onions, sliced into even half-moons about 3–4 mm thick
• About 20–25 g butter, divided between the start and the final onion glaze
• 1 tbsp neutral oil, such as rapeseed oil
• 1–2 tsp balsamic vinegar, then adjust to taste
• ½–1 tsp Henderson’s Relish, then adjust to taste
• Plenty of freshly ground black pepper and a small pinch of salt
• No sugar — they sweeten as they cook
• 2 pork burger patties (thicker patties, about 25–30 mm, work best for the reverse-sear version)
• 2 Lancashire oven-bottom muffins, not brioche rolls
• Blackjack Vintage cheddar, two generous slices for the bottom muffin halves
• 2 slices of American cheese, one for each burger patty
• A little butter for toasting the muffins
• A cast-iron or stainless-steel frying pan, a spatula and a clean instant-read food thermometer

Method

1. Read through the method first. Get the cheeses out, split the oven-bottom muffins and put the pan, spatula and thermometer within reach. The onions take longer than the burgers, so they go on first. I use cast iron or stainless steel — no PTFE or non-stick pans in my kitchen.
2. Slice the onions. Peel the three big onions, halve them from top to bottom and slice into even half-moons around 3–4 mm thick. Three enormous onions look like a ridiculous quantity for two burgers. They won’t once they’ve cooked down.
3. Start gently. Set a wide, heavy cast-iron pan over medium-low heat. Add the oil and most of the butter. Once it’s melted, add the onions and a small pinch of salt. Toss until coated, then spread them across the pan. I want a gentle sizzle, not browned edges in the first two minutes.
4. Soften before you caramelise. Give them about 15 minutes, turning every three or four minutes so the bottom doesn’t burn. They should collapse, go soft and turn translucent. If anything starts catching, lower the heat and add a tablespoon of water to loosen the sticky bits.
5. Build the proper colour. Keep them on the low-to-medium heat for another 20–30 minutes, stirring more frequently as they darken. They should shrink dramatically and turn deep golden brown, not black or crispy. I’m not adding sugar; time and the onions’ own sweetness do that job.
6. Finish the 10/10 onions. Stir in the remaining butter, a teaspoon or so of balsamic vinegar, about half a teaspoon of Henderson’s Relish and plenty of freshly ground black pepper. Let that bubble gently for about a minute until the liquid coats everything and the onions look glossy and sticky. Taste; add a little more balsamic or Henderson’s if needed. Transfer them to a warm bowl so the residual heat of the cast iron doesn’t burn them.
7. For the improved version I’ve now tried: start in the oven. The reverse sear made a real difference compared with my original oven-only attempt. For a repeatable approach, preheat the fan oven to about 130°C. Set the pork patties on a rack over a tray to catch the juices, and cook gently until a clean probe reads about 60–65°C in the thickest centre. This is only the intermediate temperature: the burgers are not cooked enough to eat yet. Timings depend entirely on their size and thickness; use the thermometer, not a fixed number of minutes.
8. Then reverse-sear in a very hot pan. Take the burgers out, drain any collected moisture and carefully pat the outsides dry. Heat a clean cast-iron or stainless-steel skillet until properly hot, add a thin film of oil and sear for about 45–90 seconds per side to create the dark, savoury crust that the oven-only burgers missed. Check the centre again: because this is minced pork, it must finish thoroughly cooked — for example, at least 75°C held for 30 seconds (or an equivalent safe time–temperature combination). If the outside is browning too fast before the centre is done, turn down the heat and continue until safe. A dark crust is not proof of a fully cooked middle.
9. Melt the American cheese. Put one slice on each burger just before it finishes, letting the residual heat soften it. Cover very briefly with a lid if it needs help. Keep the Blackjack Vintage cheddar separate: that goes on the bottom muffin, not on top of the patty.
10. Toast the proper Lancashire oven-bottoms. Cut the muffins open and toast the cut sides with a little butter in the warm pan until lightly golden. Don’t over-toast them: an oven-bottom is soft and bready, and I still want that texture. A light crust simply stops the butter and onions soaking straight through.
11. Build in the right order. Bottom half of oven-bottom muffin, a generous slice of Blackjack Vintage cheddar, hot pork burger, melted American cheese, then a magnificent pile of sticky caramelised onions. Add the top of the muffin, press it down just enough to hold together and serve immediately. Two cheeses, two different jobs: Blackjack for the rich cheddar flavour underneath, American for the melty layer above.
12. What I actually did on 13 October 2026. The photographs are of the burgers cooked completely in the oven, without the final skillet sear. I followed the burger packaging and checked that the pork was cooked through. The onions and cheese combination was excellent, but the patties released moisture and didn’t get the crust I wanted. I’ve now cooked them again using a reverse sear and the result was much better. I’m keeping the original photos because they document the first attempt; I don’t yet have a photograph of the improved batch. The oven temperature and timings above are a practical repeat guide, not a precise log of the second cook.