Oysters are a great addition to risotto, as the heat of the rice itself gently cooks the mollusks and the liquor from the oysters infuses the whole dish with sweet, briny, delicate flavor.
Sauté the onion in the butter in a large, heavy-bottomed pot over medium heat until translucent. Add the rice and toss to coat with the butter. Cook until the rice begins to toast and has a nutty aroma, 3 to 4 minutes. Add the wine and orange juice and zest and cook until there is no longer an aroma of alcohol and the rice has absorbed most of the liquid. Add the squash and water. Season to taste with salt and stir as you continue to cook over medium heat.When the rice has absorbed most of the water, about 12 minutes, add the crème fraîche and herbs. Bring to a boil, then turn off the heat and allow the rice to absorb the remaining liquid, about 5 minutes. Add the oysters and their liquor and stir to combine. The residual heat of the rice will cook the oysters; the dish is ready to serve when the edges of the oysters begin to curl, about 3 minutes. Serve immediately in shallow bowls, doing your best to distribute the oysters evenly.
Oysters can be left off and risotto served as a side dish, under slow-roasted fish. The residual heat of the rice will keep the fish warm.