Place flour, water, eggs, 2 tablespoons melted butter, and 1/2 teaspoon salt into a blender. Cover and blend for 2 minutes. Chill batter in refrigerator for two hours.
Heat an oiled, non-stick skillet over medium heat. Pour in enough of the chilled batter to cover half of the skillet. Tip and rotate the skillet until the batter covers the entire area. Cook until the batter turns from wet to moist, and the edges begin to curl away from the sides of the skillet. Turn the crepe over, and continue cooking until lightly golden on the other side. Place the crepe onto a plate, and cover with a kitchen towel to keep moist. Repeat with the remaining batter, lightly oiling the skillet as needed to keep the crepes from sticking.
Cook and stir green onions with 4 tablespoons butter in a skillet over medium-high heat until tender, 3 to 5 minutes. Add sherry, 1/4 teaspoon salt, and 1/4 teaspoon ground black pepper; stir to combine. Mix in crab.
Melt 4 tablespoons butter in a saucepan over medium-high heat. Whisk in flour. Add milk, 1/4 salt, and 1/4 teaspoon ground black pepper; bring to a boil for 1 minute. Remove from heat.
Beat heavy cream and egg yolks together in a small bowl. Gradually add egg mixture to butter-flour mixture, stirring constantly. Fold in grated cheese to form a thick sauce.
Pour sauce into crab meat mixture, reserving 1 cup of the sauce; stir to combine.
Lay out one crepe on a flat surface. Spoon 2 heaping spoonfuls of crab mixture at one end of the crepe, then roll crepe up around the filling. Repeat with remaining crepes. Top each with reserved cheese sauce and serve.