Prepare a basic crepe recipe in advance. Crepes can be made and frozen up to several weeks ahead of time.
Preheat oven to 350 degrees F (175 degrees C).
Melt butter in a saucepan over medium heat. Stir in onion, and cook 2 minutes. Stir in flour until blended. Slowly whisk in 1 cup milk, whisking constantly until smooth. Mix in broth, wine, and tarragon; stir over medium-low heat until thickened (about 4 minutes).
In a small bowl, gradually stir 3 tablespoons of the hot milk and broth sauce into the egg yolks; then pour egg mixture into sauce, whisking briskly. Cook 1 minute more, then remove from heat.
In a medium bowl, mix together half of the sauce with the chicken. Season with salt to taste.
Fill each crepe with 3 tablespoons of chicken mixture. Roll crepes, and place seam-side down in a 9x13 inch baking dish. Thin the remaining sauce with 1/4 cup milk, and pour over crepes.
Bake in a preheated oven 20 minutes, or until sauce begins to bubble.
