Heat a medium saucepan on medium-high heat. Add the oil. Saute the onions for 2 to 3 minutes. Add the tomatoes and saute for another 3 minutes, stirring frequently. Add the water and lentils; cook until most of the water is absorbed, about 20 minutes. Stir in the thyme, garlic, salt and pepper. Ragout should be fairly thick but not dry.