Place rack in lower third of oven; preheat to 350ºF.
Melt butter over medium-high heat in a large ovenproof Dutch oven; add rice and cook, stirring, until lightly toasted, about 1 1/2 minutes. (If using curry powder, add it now and cook, stirring, until fragrant, about 15 seconds.) Add water. Stir in lentils, garlic cloves, cinnamon stick, ginger, curry paste, if using, and salt; bring to a boil, stirring to dissolve the curry paste.
Cover the pot tightly with a lid or foil. Transfer to the oven and bake until the rice and lentils are tender and all the water is absorbed, 50 to 55 minutes. Fluff with a fork, removing the cinnamon stick and ginger slices. Serve garnished with scallions.