In a 6 quart pot, melt butter over low heat. Gradually add the flour, stirring constantly. Cook over low heat until a medium brown roux is formed (10 to 20 minutes).

Quickly add the onion, green pepper, celery, and garlic and continue to cook, stirring frequently, until vegetables are glazed and tender (about 20 minutes).

Add the shrimp, salt, black pepper, cayenne, lemon juice, scallions and parsley - mix well. 

Add 1 cup of cold water and bring to a boil, then lower heat and simmer until the shrimp are tender and have changed color, 4 to 6 minutes, stirring frequently.

Just before serving,  gradually add 1 to 2 cups hot water to provide the gravy. Serve over boiled rice.

Note: This makes a great one-dish meal. You can cook etouffée in advance, refrigerate it, and reheat it a portion at a time over a few days. For best results, always cook fresh rice when you reheat.