Preheat oven to 350°. Lightly grease a 13 by 9 by 2-inch baking dish.
In a large skillet, melt 2 tablespoons butter over medium heat. Add cream of mushroom soup and onion and cook for 2 minutes, Stir in sherry, fontina cheese and chicken. Cook until cheese is melted.
Spoon 1/4 cup chicken mixture down center 1/3 of each crepe. Place 2 asparagus spears over chicken mixture. Fold edges of crepe over filling, and place, seam side up, in prepared baking dish. Sprinkle 1/2 cup Swiss cheese over crepes. Bake for 10 minutes, or until cheese is melted and crepes are hot.
In a medium bowl, combine flour and salt. In another medium bowl, whisk together milk, egg, egg yolk and 1 tablespoons melted butter. Gradually add flour mixture to milk mixture, whisking until smooth.
Heat an 8-inch nonstick skillet over medium-high heat. Spoon 2 1/2 tablespoons of batter into skillet; cook for 30 to 45 seconds. Loosen crepe with a spatula, and carefully turn over. Cook for 30 to 45 seconds; transfer to plate. Repeat procedure with remaining batter, stacking crepes on a plate.