In a small skillet over medium heat, sauté onion in butter until transparent. Stir in flour and cook slowly 3 to 5 minutes. Blend in milk and egg yolk until smooth. Add salt and pepper. Cook, stirring constantly for 8 to 10 minutes or until sauce thickens. Remove from heat, add sliced eggs and mix lightly. Spoon into 2-8 oz. casseroles and sprinkle lightly with parmesan and paprika. Place in oven and heat thoroughly. Serve over toasted English muffins or Holland rusk.