1. 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.

2. 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.

3. 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 hot.