Heat the oven to 425F.
Combine the bones, onions, carrots, and celery in two roasting pans. Divide the tomato paste between the two pans. Toss and stir the ingredients until everything is fully coated in tomato paste.
Roast the two pans for about 30-35 minutes, stirring once throughout. Until everything is browned all over.
Once everything is roasted, add everything to your largest pot. Fill the pot all the way with cold water. Add bay leaves, garlic, peppercorns and rosemary to the pot.
Bring the pot just up to a boil, then immediately turn the temperature down to a low heat. Allow to simmer for 3-5 hours, until the stock has reduced by about 1/2. You can let it reduce even further if you'd like, but keep an eye on it. As it reduces, it will begin to reduce more quickly, and you don't want your pot to dry out.
When it's reduced to your liking, carefully strain into a bowl. Allow the stock to cool slightly at room temperature, then move the bowl into the fridge. Allow to cool fully. During the cooling, all the fat will harden on top, and you can remove it easily before canning, or freezing the stock.
If you want, you can re-boil the bones and veggies 1-2 more times, and get more stock. The stock will become less flavourful the more times you boil it, but getting the most out of your ingredients is always good. I usually get three rounds of stock out of a batch of bones and veggies.