French onion soup with croque monsieur croûtons

By Matt Tebbutt From Saturday Kitchen Ingredients For the French onion soup 30g/1oz butter 4 large white onions, finely sliced 100g/3½oz smoked pancetta lardons, diced 1 tsp chopped fresh thyme 2 garlic cloves, crushed 1 bay leaf 1 tbsp yeast extract, such as Marmite …