Preheat oven to 425 degrees.
Lightly oil a large roasting pan and place the bones in the pan. Cut the onions in half and then into thirds, leaving the skin and base of the onion attached. Place the onion wedges among the bones. Roast the bones and the onions in the oven for 1.25-1.5 hours or until everything is deeply caramelized brown (flavor comes with that color!).
Cut the carrots and celery into 3”-4” pieces.
Make the bouquet garni; use a large piece of cotton cheesecloth to wrap up the herbs, garlic, and peppercorns. Tie with cotton butcher’s string.
When the bones and onions are ready, transfer them to a large stock pot.
Place the roasting pan on the stove burners on medium heat and deglaze the pan with the red wine, scraping up any of the bits and pieces (the fond) stuck to the bottom of the pan. Add the tomato paste and a cup of water and whisk it into the wine to dissolve the paste. Pour all of this into the stock pot over the bones.
Add the carrots, celery and bouquet garni to the stock pot. Add cold water, about 2 gallons, to make sure everything is covered by at least 1 inch.
Bring the water to a low simmer and cook for at least 4 hours. Cook longer, up to 12 hours, for a more reduced, richer stock. Skim as needed. Add water, if needed to keep the bones covered.
When reduced to the point you want, you can pull the bones out with tongs and place in another pot if you intend to make remouillage (a second stock made from the bones used to make a primary stock. It’s a weaker stock but makes full use of the ingredients and can be used to expand the original stock or as a soup base). For a remouillage you would re-use the bones and cook them again with fresh water and vegetables.
Strain the primary stock though a fine mesh sieve or through a colander lined with cheesecloth.
The final stock will be reduced by half of the original liquid and a rich brown color. If you want demi-glace, allow it to reduce the strained stock by half again until it is thick enough to coat the back of a spoon.