Whisk all the ingredients together in a mixing bowl.
Heat a frying pan and grease it with a little butter. Add 4 tablespoons batter and swirl it to make a thin crepe.
When the underside of the crepe begins to brown (about 2 minutes) flip the crepe and cook the other side a further 2 minutes. Repeat with the remaining batter. The crepes can be made in advance and frozen for several weeks.
To make the filling:
Preheat oven to 180 degrees C.
Melt butter in a saucepan over medium heat. Stir in onion and cook 2 minutes. Stir in flour until blended and smooth. Slowly add 1 cup milk, whisking constantly until smooth. Mix in stock, 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 stock into the egg yolks; then pour egg mixture back into sauce, whisking briskly. Cook 1 minute more then remove from heat.
In a medium bowl, mix together half of the sauce with the turkey. Season with salt to taste.
Fill each crepe with 3 tablespoons of turkey mixture. Roll up crepes and place seam-side down in a 20x30cm 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.