Heat oil2 tablespoons in a medium saucepan over medium heat. Add leek1, garlic, and tomato paste1 tablespoon and cook, stirring often, until fragrant and tomato paste begins to darken, about 4 minutes. Add lentils1 cup and 2 1/2 cups water. Bring to a boil; reduce heat, cover, and simmer, stirring occasionally, until lentils are tender, 45–55 minutes.