Step 1

Stir coriander, cumin, mustard seeds, and 2 Tbsp. oil in a medium saucepan over medium-high heat until sizzling, about 1 minute. Add onion and garlic; cook, stirring often, until just softened, about 5 minutes. Add lentils and 3 cups broth; bring to a boil. Reduce heat and simmer, partially covered, adding broth as needed to keep lentils covered, until lentils are soft, 30–40 minutes. Stir in vinegar; season with salt and pepper.

Step 2

Transfer lentils to a large bowl and top with yogurt. Season with salt and pepper, drizzle with oil, and top with mint.

Step 3

DO AHEAD: Lentils can be cooked 2 days ahead. Let cool; cover and chill.