Make a dark brown roux with oil and flour. Remove 3 Tablespoons of roux and set aside for stuffed heads. In a large pot, place remaining roux; add onions, celery, and bell peppers; cook until transparent. Add water, bay leaf, lemon, salt and pepper. Simmer for approximately 3 1/2 hours. Add crawfish tails, stuffed heads, fat and green onions. Simmer for 15 minutes. Add parsley just before serving as a gumbo over cooked rice.