Preheat oven to 450F. In a large mixing bowl, toss bones with 2 tablespoons of the tallow, and scatter onto a large baking sheet, with cut faces down. In the same bowl, whisk together the remaining tallow with the tomato paste, then toss carrot and onion to coat. Scatter onto the baking sheet around the bones. Roast until everything is toasty but not burnt, about 40 minutes, flipping bones and veg halfway.
Transfer all the sheet contents—juices too—to the insert of an Instant Pot, or your largest stockpot. Fill with 1 quart water and cook on high pressure for 2 ½ hours, or simmer gently on the stove for *at least* 16 hours and up to 24. Add water as needed, if evaporating too quickly. Fine-strain out solids, then transfer to jars. After chilling overnight, the fat will have solidified and can be peeled off, if desired.