Melt the butter with the oil in a medium skillet over medium heat. Add the onions, sugar, and some salt and pepper. Cook until the onions become deep brown and sticky, about 20 minutes, stirring every 4 or 5 minutes (if the onions color too quickly, reduce the heat to medium-low).
While the onions caramelize, make the rice. Heat the remaining tablespoon of oil in a saucepan over medium heat. Add the garlic and cook, stirring, until fragrant, 30 seconds to 1 minute. Add the rice and stir, cooking it until the grains begin to turn opaque, about 2 minutes. Add the water, lentils, and cumin; season with salt and pepper. Increase heat to medium-high and bring to a boil. Reduce the heat to low, cover, and cook until the rice is tender and has absorbed all of the liquid, about 20 minutes. Serve topped with the caramelized onions.
Recipe courtesy of Ingrid Hoffmann, 2008