In a large pot melt margarine over medium heat and saute the onions, about 4 minutes. Toss in flour and stir around until mixture is smooth. Begin a steady stirring motion and gradually start pouring in the half-and half.
Keep at medium heat to prevent cream from sticking. When mixture is hot, add a few tablespoons to the egg yolks to bring the egg temperature up to keep the yolks from cooking when it is added to the rest of the sauce. Stir the yolks into the sauce until the sauce is smooth and has turned a golden color. Add salt and pepper.
Continue to stir and add cheeses. Stir until melted. Reduce heat to medium low. Finally add crawfish tails, parsley and green onions. Stir. Simmer for 2 to 3 minutes. Spoon mixture into baked pastry shells to overflowing.
Cajun Country -- New Iberia, Louisiana