Remove the tails and peel, preserving the meat and peelings. Clean the head section, using your index finger to remove cartilage and membranes. Pinch off the claws and reserve. Rinse the cleaned heads (you should have about 100) in cool water and soak in cool water for 15 minutes. Drain and pat dry. Set aside.
Put the tail peelings and claws in a stockpot and cover with the water. Bring to a boil. Simmer, uncovered, for 45 minutes. Drain. You should have about 3 quarts of stock. Let stock cool.
Melt the butter in a large skillet over medium-high heat. Add the onions, bell peppers, celery, salt, and cayenne and sauté for 6 to 7 minutes, or until the vegetables are soft and golden. Add the crawfish tails and the garlic. Cook, stirring occasionally, for 8 to 9 minutes, or until slightly golden. Add the water and simmer for 2 minutes. Remove from the heat and put the mixture into a mixing bowl. Add the bread crumbs and parsley and mix well. Let cool.
Preheat the oven to 350ºF.
Stuff each head with about 2 tablespoons of the stuffing. The amount will vary depending on the size of the heads. Reserve any remaining stuffing mixture. Dredge the heads in flour and place on a baking sheet. Bake for 30 minutes.
Combine the oil and flour in large cast-iron or enameled cast-iron Dutch oven over medium heat. Stirring slowly and constantly for 20 to 25 minutes, make a medium brown roux, the color of peanut butter.
Add the onions, bell peppers, celery, salt, and cayenne. Cook, stirring often, 6 to 7 minutes, or until the vegetables are soft. Add the crawfish tails. Stir and cook for 3 to 4 minutes. Add the crawfish stock and bring to a boil. Simmer over medium heat, uncovered, for about 1 hour 15 minutes. Add the reserved stuffing mix, stuffed crawfish heads, and cook, stirring occasionally, for 15 minutes. Add the green onions and parsley.
Serve in deep soup bowls. Remove the stuffing from the heads with your fork.
Yield: 10 to 12 main-course servings