1 teaspoon ground cumin
1/2 teaspoon ground coriander
1/2 teaspoon ancho chile powder
1 pound tomatoes, seeded and diced
2 cups red lentils (14 ounces)
Salt
Plain yogurt, lemon wedges and warm pita, for serving
2 tablespoons unsalted butter
1 medium onion, chopped
2 carrots, finely chopped
3 celery ribs, finely chopped
3 garlic cloves, thinly sliced