In a large saucepan, heat the oil over medium heat. Add the onion and garlic and cook until softened but not browned, 2 to 3 minutes. Add the remaining ingredients, except the tomatoes, and bring to a boil. Reduce heat to low, cover, and simmer until the lentils are tender, 25 to 30 minutes. Add the tomatoes and simmer 5 more minutes.
