In a large pot, heat olive oil over medium heat and sauté onions, garlic, leeks, celery and carrots for 5 to 10 minutes, until lightly browned (optional). Add cold water, tomato and bouquet garni. Bring to the boil, then reduce heat to low and simmer for about 1 hour, skimming the surface if necessary. Season with salt and pepper.
Filter the broth through a fine sieve and use immediately, or store in the fridge or freezer.
This broth is a versatile base for soups, risottos or vegetarian sauces!