Preheat the oven to 350?F.
Melt the butter in a large ovenproof saucepan or Dutch oven over medium-high heat. Add the onions and cook, stirring, until soft, 3 to 4 minutes. Add the rice and cook, stirring, until opaque and nutty in aroma, 2 to 3 minutes. Add the chicken broth and salt, stir well, and bring to a boil. Cover with a tight-fitting lid and transfer to the oven. Bake until the rice is tender and the liquid is absorbed, 25 to 30 minutes.
Remove the rice from the oven and let stand, undisturbed, for 5 to 10 minutes. Fluff the rice with a fork and stir in the green onions, if desired. Serve hot.
Makes about 8 cups, 8 to 12 servings