1 cup butter, divided
2 large eggs
3 tablespoons milk
1 large egg yolk, beaten
1 teaspoon white sugar
1 teaspoon vanilla extract
1 teaspoon lemon zest
2 teaspoons lemon juice
1 small onion, sliced into 1 inch rounds
Preheat oven to 350 degrees F (175 degrees C).
In a medium mixing bowl, beat margarine and flour. Stir eggs and milk into margarine mixture just until blended. Pour egg mixture into pan and spoon mixture onto every single slice.
Place first layer of slices on a baking sheet. Spread with egg mixture. Flatten with water and sprinkle with lemon zest. Spread with lemon juice. Place second layer of slices on top of egg mixture. Spread with lemon juice. Layer with remaining slices and lemon juice. Top with lemon cake, lemon frosting, lemon frosting sandwich, loaf or other bread, or bread sandwich sprinkled with lemon cake.
Bake in preheated oven for 60 minutes. Cool slightly. Serve at room temperature.