First, make a roux. Melt butter in a large iron skillet over medium heat. Using a wooden spoon, stir in the flour for several minutes until medium brown in color.
Lower the heat slightly; add the onion, bell pepper, & celery and saute until they begin to soften.
Add crawfish tails, water, tomato paste, green onions, & seasonings to suit your taste.
Lower the heat and gently simmer for about 30 mins, adding a little more water, if needed, to thin it out.