Step 1: Melt the butter in a heavy-bottomed skillet or Dutch oven over medium heat. Gradually whisk in the flour to create a roux. Stir constantly for 12-15 minutes until the roux reaches a light brown or peanut butter color.
Step 2: Add the diced onion, bell pepper, and celery (the Holy Trinity) to the roux. Sauté for 5-7 minutes until the vegetables are soft and translucent. Add the minced garlic and cook for another minute until fragrant.
Step 3: Slowly pour in the seafood stock while whisking continuously to prevent lumps from forming. Add the Cajun seasoning, cayenne, salt, and pepper. Bring the mixture to a gentle simmer.
Step 4: Reduce heat to low and let the sauce thicken for about 10 minutes. Fold in the crawfish tails and any juices/fat from the package. Simmer for another 5-8 minutes until the crawfish are heated through.
Step 5: Stir in the green onions and parsley. Taste and adjust seasoning as needed. Serve hot over a bed of fluffy white rice.