To make a rich roasted vegetable stock, omit the butter and leeks. Increase the quantity of onions to 900 g (2 lb) and carrots to 450 g (1 lb). Instead of sweating the onions as in step 1, roast them with the carrots. Preheat the oven to 220°C (425°F, gas mark 7) and toss the onions and carrots with 1 tbsp extra virgin olive oil in a flameproof casserole dish. Roast the vegetables for 40 minutes, stirring twice. Add the remaining ingredients and follow steps 2 and 3 to finish the stock. To further enrich the stock, add 10 g (¼ oz) dried porcini mushrooms and 1 tbsp dry sherry with the water.