Put the oil in a large skillet or flameproof casserole with a lid over medium-high heat; a minute later, add the onion and cook, stirring occasionally, until it begins to brown on its edges, about 5 minutes. Stir in the curry powder, then 3 cups water. Bring to a boil and add the lentils and rice. Bring back to a boil, then cover and adjust the heat so the mixture simmers steadily.
Cook until the rice and lentils are tender, about 20 minutes. Uncover and, if necessary, boil off excess water. Season with salt and pepper, then stir in the butter if you’re using it. When it has been incorporated, turn off the heat and serve.
