Cut the onion in half and stick a clove into the round part of each half.
Then prepare a bouquet garni by tying the bay leaf, thyme, and parsley stems inside the leek leaf.
Place the meat, celery, carrot, the two onion halves, any bones, and the tomato into a large pot, and cover with the indicated amount of cold water.
Heat the pot over a low temperature between 37°C and 67°C (99–153°F) to ensure maximum flavor extraction (a thermometer can be helpful). Temperatures up to 95°C (203°F) can also be suitable, as long as the water does not boil.
Cook the broth for about 2 hours, skimming off any foam that forms on the surface from time to time.
After the cooking time, strain the broth first through a fine-mesh sieve and then through cheesecloth or muslin.
Finally, adjust with salt and pepper to taste and serve hot.