Preheat an oven to 375 degrees F (190 degrees C). Prepare a 1 quart baking dish with butter.
Melt the butter in a large skillet over medium-high heat. Fry the onion in the butter until translucent, 3 to 5 minutes. Stir in the rice; cook until the rice is slightly golden. Pour in the chicken broth. Season with the salt. Simmer another 5 minutes. Transfer to the prepared dish.
Bake in the preheated oven until the broth is completely absorbed, 35 to 40 minutes. Fluff with a fork to serve.