Petit Trois's French Onion Soup

Preheat oven to 425°F. Place a wire rack inside a rimmed baking sheet and place bones on top. Roast bones (undisturbed) until deep golden brown, 40–50 minutes. Transfer to a large pot and add 5 quarts of water. Bring to a simmer, then use a fine-mesh sieve to skim off any foam or gray bits from the surface. Remove from heat.

Meanwhile, in another large pot, heat oil over medium-high. Add onions, celery, and carrots and cook, stirring occasionally, until vegetables start to caramelize, 10–15 minutes. Add tomato paste and cook, stirring often, until slightly darkened in color, 6–8 minutes.

Add bones along with liquid, garlic, thyme, and bay leaf to vegetables. Bring just to a boil; reduce heat to low, and cook at a bare simmer, adding water as needed during cooking to maintain level of liquid until stock is flavorful, about 2 hours. Strain stock through a fine-mesh sieve into a large bowl, then strain again into another large bowl or airtight containers.