Place the carrots, onion, garlic, and crushed red pepper, if desired, in a medium microwave-safe bowl. Add 2 tablespoons water and cover bowl with waxed paper. Microwave on 100% power for 4 minutes.
Coat the inside of a 4-quart slow cooker with cooking spray. Add the onion mixture, lentils, broth, and salt to cooker; stir to combine. Cover and cook on low-heat setting for 5 1/2 hours.
Stir in the oregano, rosemary, lemon peel, lemon juice, and pepper to taste.