Combine broth and lentils in a medium saucepan; bring to a boil. Reduce heat, partially cover, and simmer 10 minutes or until lentils are tender. Remove from heat; cover and set aside.
Heat oil in a large nonstick skillet over medium heat. Add onion, garlic, mustard seeds, and red pepper, and cook 5 minutes or until onions are tender and seeds begin to pop, stirring constantly. Add cumin, turmeric, coriander, and black pepper; cook 3 minutes, stirring constantly. Add tomato paste, and cook 3 minutes, stirring constantly. Add lentils, coconut milk, and salt; cook 3 minutes, stirring frequently. Remove from heat; stir in juice. Cool to room temperature. Serve dal with pita wedges.