Rinse rice (we prefer long grain brown rice) and lentils thoroughly in three changes of water. Drain well.
In a large saucepan over medium heat, melt the butter. Add the drained rice and lentils and toss until well-coated.
Add broth and bring to a boil. Reduce to low heat and simmer, covered, undisturbed, for 30-40 minutes, until broth is mostly absorbed and rice and lentils are tender.
Remove from heat and let rest 5-10 minutes before fluffing with a fork. (Avoid stirring vigorously, as that creates too creamy a texture. The rice and lentils are best when the grains are tender, but separate.)