Caramelised Onion Soup with Gruyère Toasts

Soften the onions. Melt the butter in the pot over medium-high heat, then add the onions, salt and sugar. Cover and cook for 15-20 min, stirring every few minutes, until the onions have collapsed and released their liquid. The lid traps steam, which wilts the onions quickly before browning begins.

Caramelise slowly. Take off the lid, lower the heat to medium and cook for 35-45 min more, stirring every 3-4 min and scraping the brown film from the bottom. The onions are ready when deep golden-brown and jammy. If they start to catch, add 2 tbsp water and scrape; that brown film is concentrated sugar and flavour, so you want it back in the onions, not burnt.

Add flour and stock. Stir in the flour for 1 min, then pour in 250 ml of the stock and scrape the bottom clean. Add the rest of the stock with the water, thyme, bay leaf and pepper. Bring to the boil and simmer uncovered for 20 min; the flour gives the broth a little body.

Season and brighten. Lift out the thyme stalks and bay leaf, then stir in the lemon juice. A touch of acid balances the sweetness of the caramelised onions.

Toast the bread. Meanwhile, heat the oven grill (broiler) to high. Toast the baguette slices on the tray under the grill for 1-2 min per side until dry and golden. Dry toasts float on the soup and hold up under the cheese rather than going soggy straight away.

Gratinate and serve. Set the bowls on the tray, ladle in the boiling-hot soup and float 2 toasts on each, then cover with the cheese. Grill for 3-5 min until the cheese is melted, bubbling and browned in spots. The bowls will be very hot, so lift them with oven gloves and serve on plates.