Bring water to a boil in a pot and stir in lentils. Reduce the heat and simmer 20 minutes; drain.
Heat olive oil in a large skillet over medium heat. Add bell pepper and onion and cook, stirring, until tender. Mix in tomatoes and season with salt and pepper. Stir in lentils, reduce the heat, and simmer until lentils are tender, 25 to 30 minutes.