Using a vegetable peeler (preferably a Y-style peeler), remove the zest from 1 of the lemons in long, wide strips; try to remove only the colored portion of the peel, not the bitter white pith just underneath. Using a rasp-style grater, grate the zest from the remaining lemon; set aside separately. Halve the lemons and squeeze ¼ cup juice; set the juice aside.

In a medium saucepan over medium, heat 2 teaspoons oil until shimmering. Add the shrimp shells and cook, stirring constantly, until pink, 1 to 2 minutes. Add 5 cups water, the zest strips and ½ teaspoon salt, then bring to a simmer. Cover, reduce to low and cook for 10 minutes. Pour the broth through a strainer set over a medium bowl; rinse out the pan. Press on the solids to extract as much liquid as possible, then discard. Return the broth to the pan, cover and set over low to keep warm.

In a large Dutch oven over medium-high, heat 1 tablespoon of oil until shimmering. Add the onion and ¼ teaspoon salt, then cook, stirring occasionally, until softened, 6 to 7 minutes. Add the rice and cook, stirring, until the grains are translucent at the edges, 1 to 2 minutes. Add the wine and cook, stirring occasionally, until the pan is almost dry, about 3 minutes. Add 3 cups of the hot broth and cook, stirring often and briskly, until a spoon drawn through the mixture leaves a trail, 10 to 12 minutes.

Add the remaining broth and cook, stirring, until the rice is tender, 8 to 10 minutes. Remove the pot from the heat and stir in the shrimp. Cover and let stand until the shrimp are opaque throughout, 5 to 7 minutes.

Stir in the remaining 1 tablespoon oil, the lemon juice, egg yolk, cream, basil, and the grated zest. The risotto should be loose but not soupy; if needed, stir in water 1 tablespoon at a time to achieve the proper consistency. Taste and season with salt. Serve drizzled with additional oil.