Lop the ends off the grapefruit and the lime. Use a paring knife to remove the skin and cut the segments out over a bowl, being sure to save any citrus juices as well. If there are any tough bits of membrane attached to any citrus segments, remove them with a knife. Squeeze the membranes over the bowl after sectioning to extract as much juice as possible.: The moment you start handling the grapefruit and lime , you will notice their bright, fragrant oils; that scent alone lifts your mood. As you remove the skin and cut the segments, let the juice collect in a bowl because that liquid is concentrated flavor that will fold into the risotto. Sensory cues to watch for include firm but yielding fruit segments and a bowl that begins to glisten with juice. A helpful technique is to work over a shallow bowl so you can see and salvage every drop. One common mistake is rushing and leaving behind membranes that taste bitter, so take your time to remove any tough bits carefully. If your paring knife slips, stop and reposition the fruit to keep cuts clean and controlled.
Heat the butter over low-to-medium heat in a medium saucepan (4-6 quarts/liters.) Add the onions, season with a pinch of salt, and cook, stirring frequently, until the onions are translucent, about 5 minutes. Add the rice and stir until the rice is coated with butter and shiny. While you are cooking the onions and rice, heat the stock in a saucepan and keep warm while you continue making the recipe.: As the butter melts and the onion softens, you will smell a gentle sweetness, and the sound of the pan will shift from a hiss to a softer sizzle. The pan should feel warm but not aggressively hot, because gentle heat coaxing the onion into translucence preserves its sweet notes without browning. When you add the rice , each grain will take on a glossy sheen from the fat, which is the visual sign you want before any liquid touches it. Keeping the stock warm nearby is essential, because adding cold liquid cools the pan and interrupts the cooking rhythm. A frequent error is overheating and browning the onion, which introduces caramelized flavors that alter the intended profile, so keep the flame moderate and stir regularly.
Stir about 2 cups (500 ml) of the stock into the rice. Keep the heat at a gentle simmer and as the first amount of stock gets absorbed, while stirring occasionally, add another 1 cup (250 ml) or so, stirring as needed, to keep the rice cooking and absorbing the liquid evenly. Continue to add more stock, about 1/2 cup (125 ml) or so at a time, seasoning the risotto with salt when the rice is very close to being fully cooked. At that point, add the citrus sections and juice to the risotto, crumbling them roughly as you add them to the risotto. Gently cook the risotto, continuing to stir occasionally until the rice is no longer firm. It's hard to say exactly how much stock the rice will absorb, but it's done when the rice is soft and velvety, and no longer tastes raw. I used the full 5 cups of stock but yours may require less.: When you pour the hot stock in, listen for the satisfying hiss as the liquid meets the warm rice, then settle into a gentle simmer. The first additions are about coaxing starch out of the rice so it becomes creamy, while subsequent smaller additions control texture. Stirring helps distribute heat and promotes even absorption, and you will see the surface go from glossy to slightly thickened as starch releases. The right doneness is when the grains are plump and tender with a slight bite in the center, not chalky. Be mindful of over stirring which can make the risotto gluey, and avoid adding all the stock at once, which prevents proper release of starch and ruins texture. If the risotto seems too dry midway, add a splash of stock and give it time to relax.
Remove from heat and stir in the mascarpone. Divide into soup bowls, mounding the risotto in the center of the dish. Serve with shrimp, fish, or vegetables, as desired.: At the finish, when the rice feels tender and the pan has been pulled from the heat, the mascarpone is folded in to enrich and smooth the texture, and you will immediately notice the surface become silkier and more cohesive. The aroma should be a balance of warm, savory stock and lifted citrus. Spoon the risotto into shallow bowls so each portion cools slightly and the textures hold. If you plan to serve alongside shrimp or fish , coordinate their timing so everything is hot together. A typical misstep is leaving the risotto unattended too long off heat, which causes it to thicken and stiffen; if that happens, loosen with a splash of warm stock just before serving to restore the correct consistency.