Makes about 3 cups, active time 20 minutes, total time 40 to 45 minutes
1 small carrot, peeled and chopped into rough 1/2-inch pieces (about 1/2 cup)
1 small rib celery , chopped into rough 1/2-inch pieces (about 1/2 cup)
1 small onion , chopped into rough 1/2-inch pieces (about 3/4 cup)
3 tablespoons unsalted butter
1/4 cup unbleached all-purpose flour
2 cups low-sodium chicken broth
2 cups low-sodium beef broth
1 bay leaf
1/4 teaspoon dried thyme
5 whole black peppercorns
Freshly ground black pepper
In food processor, pulse carrot until broken into rough 1/4-inch pieces, about five 1-second pulses. Add celery and onion; pulse until all vegetables are broken into 1/8-inch pieces, about five 1-second pulses.
Heat butter in large heavy-bottomed saucepan over medium-high heat; when foaming subsides, add vegetables and cook, stirring frequently, until softened and well browned, about 7 minutes. Reduce heat to medium; stir in flour and cook, stirring constantly, until thoroughly browned and fragrant, about 5 minutes. Whisking constantly, gradually add broths; bring to boil, skimming off any foam that forms on surface. Reduce heat to medium-low and add bay leaf, thyme, and peppercorns; simmer, stirring occasionally, until thickened and reduced to 3 cups, 20 to 25 minutes.
Strain gravy through fine-mesh strainer into clean saucepan, pressing on solids to extract as much liquid as possible; discard solids. Adjust seasonings with salt and pepper. Serve immediately or store in an air-tight container and refrigerate for up to a week or freeze; reheat before serving.