Heat the oil in a large saucepan over medium-high heat. Add the scallion whites and garlic and cook, stirring often, until softened, 1 to 2 minutes. Add the rice and cook, stirring often, until the grains are opaque, 2 to 3 minutes. Add the broth, tomato sauce, chili powder, 1 teaspoon salt, and ¼ teaspoon pepper and stir to combine. Bring to a simmer and cook, covered, until the rice is tender, 25 to 30 minutes.
Remove the rice from heat and let stand, covered, for 5 minutes. Fluff with a fork and fold in the scallion greens before serving.
