In a small skillet, toast whole cumin and coriander seeds until fragrant; grind in a coffee grinder. Heat oil in large pot. Add onion, garlic, carrots, celery and bell pepper. Saute until onions are softened, about 3-4 minutes. Add ground cumin and coriander seeds, chili powder, oregano, mustard and cayenne pepper. Saute another minute. Add tomato sauce and chopped tomatoes; season to taste with salt. Transfer mixture to slow cooker and add lentils and broth. Cook on high for 8 hours.