In saucepan cook celery in butter; blend in flour, salt, mustard and pepper. Add milk and bouillon granules. Cook and stir till thickened and bubbly. Stir in mushrooms; set aside. Cook the frozen asparagus according to package directions; drain thoroughly.
Reserve 1/2 cup asparagus and 1 egg for garnish. In a 10"x6"x2" baking dish arrange remaining asparagus and egg slices. Pour sauce over all. Bake, covered, at 375F for 15 minutes. Arrange reserved asparagus and sliced egg atop casserole; sprinkle with crushed crackers. Bake, uncovered, 10 minutes longer.