Melt butter in a large skillet or Dutch oven over medium heat
Add chopped onion, bell pepper, and celery
Cook until softened
Add minced garlic and cook briefly
Stir in flour to make a roux
Cook the roux until light brown
Slowly whisk in seafood stock or chicken stock
Stir until smooth
Add Cajun seasoning, salt, black pepper, and cayenne if desired
Add tomato paste if using
Simmer until the sauce thickens
Stir in crawfish tails and any reserved crawfish fat
Cook until the crawfish are heated through
Add chopped green onions and parsley
Serve over hot cooked rice
Garnish with extra green onions if desired