Season crawfish tails with salt and pepper and set aside. Melt butter in a heavy pot. Add onions, bell pepper, garlic, and celery and cook until vegetables are wilted, stirring constantly. Add crawfish fat and 1/2 cup water and cook covered over low heat for 15 minutes, stirring occasionally. Add crawfish tails and a cup of water, bring to a boil and simmer for 20 minutes. Stir occasionally. Dissolve cornstarch in remaining 1/2 cup water; add to mixture. Add green onions and parsley, season to taste with salt, pepper, cayenne, a dash or Worcestershire sauce (and hot sauce, if desired), and cook 10 minutes. Adjust liquid if necessary for thick consistency. Cover and let stand 10 minutes. Serve over hot, fluffy rice.
A teaspoon or so of tomato paste is used for color if crawfish fat is unavailable. However, there is no substitute for the crawfish fat for flavor.
Cajun Country -- New Iberia, Louisiana