Sauté the onion: In a 4-quart saucepan or another medium pot over medium- low heat, melt the butter. Add the onion and a few pinches of salt and cook, stirring regularly with a wooden spoon, until the onion is tender and translucent, about 6 minutes.

Simmer the rice: Add the rice and stir until the grains are warm and glossy. Pour in about 2 cups of the stock, adjusting the heat to let it gently simmer, then stir occasionally until it has been mostly absorbed. Add another cup or so of stock and repeat. The risotto should be starting to look like a porridge. Taste and adjust the seasoning—the rice will still be hard and a little raw tasting. Add another 1/2 cup or so of stock and stir as needed until just absorbed. Taste and adjust the seasoning again.

Stir in the citrus: Break the citrus sections into irregular chunks as you add them to the risotto. Add the grapefruit juice—if your grapefruit was very juicy, you may not need much of the remaining stock.

Beat in the mascarpone and eat: Taste again: If the rice is still quite firm, add more stock, a tablespoon at a time, and cook until the rice is still just a little firm in the center (al dente). Turn off the heat and aggressively stir in the mascarpone until the risotto is creamy and the citrus is broken down into pretty flecks. Serve immediately.