In a large pot, melt the butter over medium-high heat. Add the flour and cook, stirring, to make a light roux. Add the onions, celery, bell peppers, green onions, garlic, bay leaves, salt, and pepper and cook, stirring, until the vegetables are soft, about 5 minutes.
Add the sherry and cook for 2 to 3 minutes. Add the stock and crawfish tails and bring to a boil. Reduce the heat and simmer until thickened, about 5 minutes. Add the lemon juice.
Stir in the parsley and remove from the heat.
Adjust the seasoning, to taste. Serve over rice, garnished with additional parsley.