Position rack in center of oven and preheat the oven to 350 degrees F.

In a medium saucepan, melt the butter over medium heat. Add the onion and cook, stirring, until soft, about 3 to 4 minutes. Add the rice and cook, stirring, until opaque and nutty in aroma, about 2 minutes. Add the water, chicken broth, and salt and bring to a boil. Cover the pan with a tight-fitting lid. Using oven mitts or potholders, transfer to the oven, and bake until the rice is tender and the liquid is absorbed, 25 to 30 minutes.

Using oven mitts or potholders, remove the pan from the oven and add the peas and scallions, if desired. Toss gently to combine, then replace the cover and allow the rice to sit, undisturbed, for 5 to 10 minutes. Serve hot.