Stir coriander, cumin, mustard seeds1/2 teaspoon, and 2 tablespoons oil2 tablespoons in a medium saucepan over medium-high heat until sizzling, about 1 minute. Add onion1 and garlic; cook, stirring often, until just softened, about 5 minutes. Add lentils1 cup and 3 cups broth3 cups; 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 vinegar1 teaspoon; season with salt and pepper0.