Use a saucepan with a tight fitting lid to "Etouffe". Season crawfish tails with salt and pepper; set aside. Melt butter, add chopped onion, and cook over medium heat until onions are tender. Stir in flour, blend well, and add 3/4 cup water, lemon, tomato paste, and cook slowly for about 20 minutes, and add a little more water occasionally. When sauce is done, add crawfish tails; cover with lid. Cook 8 minutes. Season again, to taste. Add green onion, parsley, cook 2 minutes longer. Serve over steamed rice.