Melt butter in large skillet or Dutch oven and saute green onions until tender; about 10 to 15 minutes. Add parsley, crawfish tails, crawfish fat (or butter), and salt and pepper to taste. Cook over medium heat 15 to 20 minutes. If a thicker gravy is desired, dissolve cornstarch in a small amount of water and add to sauce. Serve over steamed rice and garnish with lemon slices, if desired.