How to Make It
Preheat the oven to 375°. On a rimmed baking sheet, toss the mushrooms with 2 tablespoons of the olive oil and season them with salt and pepper. Scatter half of the butter around the mushrooms and roast for 20 to 25 minutes, until they are tender and lightly browned.
Meanwhile, in a medium saucepan, combine the fish stock with 2 cups of water and bring to a simmer. Keep warm over very low heat.
In a large saucepan, heat the remaining 1/4 cup of olive oil. Add the onion and a generous pinch of salt and cook over moderate heat, stirring, until softened, about 3 minutes. Add the rice and cook, stirring, until well coated with oil, about 1 minute. Add the wine and cook, stirring, until absorbed. Add 1 cup of the warm stock and cook, stirring constantly, until nearly absorbed. Continue adding the warm stock 1/2 cup at a time, stirring constantly and allowing the stock to be nearly absorbed between additions. The risotto is done when the rice is al dente and suspended in a thick, creamy sauce, about 20 minutes total.
Stir the mushrooms, the 2 tablespoons of Parmigiano and the remaining 2 tablespoons of butter into the risotto and season lightly with salt and pepper. Gently fold in the crabmeat; stir in 1 or 2 tablespoons of water if the risotto is too thick. Spoon the risotto into shallow bowls and garnish with the chervil. Pass additional cheese at the table.