Caramelised Onion & Mature Cheddar Soup

Ingredients

• 900 ml good chicken or vegetable stock
• 1 tbsp plain flour
• salt and black pepper
• 1.2 kg white onions, peeled and thinly sliced
• 2 sprigs fresh thyme
• 1 tsp soft brown sugar
• 50 g unsalted butter
• 1 tbsp olive oil
• 1 tsp Dijon mustard
• 200 ml dry cider
• 150 g mature cheddar, coarsely grated
• 4 thick slices sourdough
• Natural yoghurt, to serve

Method

1. Melt the butter and olive oil in a large heavy pot over medium-low heat. Add the onions and leek with the brown sugar and a generous pinch of salt — season now, while the onions can absorb it, not at the end when they've already set their flavour. Cook, stirring every 5–10 minutes, for 50–60 minutes until everything is deeply caramelised, jammy and a rich mahogany brown, with sticky bits catching on the base of the pot. Don't rush this — pale onions taste of nothing, and this slow surrender is where the whole soup's flavour lives.
2. Stir in the flour and Dijon mustard and cook for 2 minutes, scraping the sticky fond up from the base as you go — the raw flour smell should disappear and the paste should darken a shade. Pour in the cider, let it hiss against the hot pot, and bubble hard for 3 minutes to cook off the alcohol and lift every last caramelised scrap off the bottom. That fond is half the flavour; leave none behind.
3. Add the stock, lentils and thyme. Bring to a gentle simmer and cook for 25 minutes, stirring occasionally, until the lentils have completely collapsed into the broth and thickened it to a velvety body — they're doing the work of cream without the weight. Fish out the thyme stalks.
4. Take a stick blender to roughly half the soup directly in the pot — you want a silky, glossy body with plenty of soft onion strands still running through. Taste, season, taste again: salt to wake the sweetness, black pepper for warmth. Adjust now, not at the table.
5. Meanwhile, heat the grill to high and toast the sourdough slices on one side until golden. Flip, pile the cheddar generously on top, and grill until bubbling, blistered and golden at the edges — watch it, the line between molten and burnt is about 30 seconds.
6. Ladle the hot soup into warmed bowls, float a cheesy cheddar toast on each, drizzle with peppery extra virgin olive oil and finish with a heavy crack of black pepper at the table.