In a blender, whirl until smooth 1/2 cup milk, 1 large egg, 1 large egg yolk, and 1/3 cup all-purpose flour.
Place a 6- to 7-inch crepe pan or other flat-bottomed frying pan on medium heat. When hot, add 1/4 teaspoon butter or margarine (you'll need about 2 teaspoons total) and swirl to coat surface.
Stir batter and pour about 2 tablespoons into pan; quickly tilt pan so batter flows over entire flat surface. Cook until surface is dry and edge is lightly browned. Turn out onto paper towels, stacking them as made. (If made ahead, cool, cover, and chill up to a week. To use, let warm to room temperature before separating; they tear easily when cold.)
8 crepes, 1/2 pound firm-ripe brie or camembert, thinly sliced 2 small Red Delicious apples, cored, quartered, and thinly sliced 1/2 cup walnut halves or pieces 2 tablespoons butter or margarine, melted
Place 4 of the crepes slightly apart in a single layer on two 10- by 15-inch baking sheets. Arrange cheese and apple evenly over crepes. Top each with remaining crepes. Mix nuts with butter and garnish crepes with nuts. Bake, uncovered, in a 425 F oven until cheese melts, about 5 minutes. With a wide spatula, transfer each crepe sandwich to a warm plate. Serve at once.