Whisk together flour and oil in a heavy saucepan over medium heat. Cook, stirring, until light brown; add onion and bell pepper and cook, stirring occasionally, until softened, about 10 minutes. Add enough water to thin slightly and cook for another 20 minutes, stirring and adding water as needed. Stir in crab and cook for 15 minutes. 

Preheat oven to 350˚F.

Process bread in a food processor to a fine crumb and add to crab mixture along with parsley and green onion. Season to taste with salt, pepper, and cayenne. Stuff into cleaned crab shells on a baking sheet or a buttered casserole dish; transfer to the oven and bake until lightly browned, 15 to 20 minutes. Serve hot.