Combine the lentils, diced mushrooms, bell peppers, chopped tomatoes, and vegetable stock in the slow cooker, stirring well to mix everything evenly. Set the slow cooker to high and let the mixture cook for approximately 3 1/2 hours, or until the lentils are tender. Stir in the grated cheddar cheese until fully melted, allowing the lid to remain on for a few extra minutes if necessary. Adjust seasoning with salt and black pepper as desired. Serve warm, optionally garnished with finely grated cheese and a sprinkle of semi-dried parsley