Fonds bruns de veau and, in some countries where veal is expensive and difficult to obtain, fonds de boeuf (beef), are made by coloring the trimmings (nerves, some fat) and bones in a very hot oven (pincer les os) until well roasted and the bones are nicely browned, being careful not to overcolor.
An aromatic garnish can be used and would include a mirepoix of leek, carrot, onion, and celery, along with a whole head of garlic, tomato paste, cut fresh tomatoes, thyme, bay leaf, and whole peppercorns. These would be added three-quarters through the roasting time of the bones and allowed to brown as well. This process adds additional color as well as flavor to the stock.