Melt butter in a large pot or skillet over medium heat.

Add chopped onions, celery, and garlic to the melted butter and sauté for 20 minutes, or until the vegetables are softened and fragrant.

Stir in parsley.

Add crawfish tails to the pot and cook for 15 minutes, stirring occasionally.

In a separate bowl, dissolve flour in half and half, ensuring no lumps remain.

Add the half and half mixture to the pot with the crawfish tails and vegetables.

Stir in the jalapeno cheese (or Velveeta) until melted and smooth.

Add cooked fettuccini to the crawfish mixture and stir to combine, ensuring the pasta is well coated with the sauce.

Pour the mixture into a baking dish.

Bake in a preheated oven at 350°F (175°C) for 30 minutes, or until bubbly and heated through.

Garnish with fresh parsley before serving.