Heat the butter in a shallow casserole dish until melted and sizzling. Add the onions and season generously with salt, then stir well until the onions are coated in the butter and sizzling gently. Tie the thyme and bay leaves together using kitchen twine and nestle this into the onions. Cook on a medium heat for around 40-45 mins. At first the onions will release lots of liquid but you want to bubble that down and after about 20 mins they will start to caramelise. This is when you need to keep an eye on them and keep stirring and scraping up any sticky bits off the pan with a wooden spoon back into the onions.

When the onions have cooked down and are sticky and brown, stir through the yeast extract, then scatter over and stir through the flour. Cook the flour for 3-4 mins, then splash in the white wine, bubble for another 3-4 mins, then stir in the stock. Simmer gently for 20 mins until you have a glossy onion soup. Taste for seasoning.

While the soup is simmering, heat the oven to 200C/180C fan/gas 6. Drizzle half the oil over a baking tray, arrange the slices of baguette in the tray, then rub each one with the cut side of the garlic and drizzle with the rest of the oil. Scatter over about a third of the cheese and bake for 8-10 mins until golden. Remove from the oven. The soup and the croûtes can be made the day before. Keep the soup chilled and the croûtes in an airtight container. Leave to cool first.

When ready to serve, turn the oven up to 240C/220C fan/gas 9. Remove the herb bundle from the soup and, off the heat, arrange the croûtes over the top of the soup and scatter over the rest of the cheese. Bake the soup for 10 mins until the cheese has melted and the soup is bubbling around the edges. Bring to the table and serve straight from the dish. Will keep frozen for up to six months. Defrost thoroughly before gently reheating.