Finely chop the vegetables except the garlic, and leave to brown in a casserole dish with a drizzle of oil. Add the well-rinsed lentils and sauté over a high heat. Add a little tomato sauce, remove the clove of garlic and add enough water to cover all the ingredients. As soon as the mixture starts to boil, add salt and pepper and reduce the heat. Cover and leave to cook for about 40 minutes, stirring frequently.