Wash the crosnes by soaking them in water and scrubing gently with a vegetable brush, taking care to get all dirt and sand out of the nooks and crannies.
Heat olive oil over medium-high heat in a heavy pan. Add the crosnes, season with salt and pepper, stir, and cook for 5 minutes. Add butter and water, reduce heat to a simmer, and cook for about 10 more minutes or until tender. Add fava beans and heat through. Remove from heat and drizzle with truffle oil.
Heat olive oil in a medium skillet. Dredge snapper in flour and saute until cooked through and slightly golden, about 3 minutes per side.
Divide favas and crosnes among 2 plates. Top with snapper and serve.
To prepare fresh fava beans: With your fingers, pop open the bean pods along their seams and pull out the individual fava beans. Use a thumbnail to split open the tough outer skin of each bean, then peel it off with your fingertips.