In a pot, bring the water to the boil, and stir in the lentils. Reduce heat, and simmer 20 minutes; drain.
Heat olive oil in a large frying pan over medium heat, and sauté the green pepper and onion until tender. Mix in the tomatoes, and season with salt and pepper. Stir in the lentils, reduce heat, and simmer 25 to 30 minutes, until the lentils are tender.