Place butter or margarine in a 9x13 inch baking dish and melt butter in oven. Remove when melted, approximately 3 minutes.
In large mixing bowl, beat eggs well. Mix in flour, milk and baking powder. When the dough is well combined, mix in cheese, spinach, and onion. Spoon mixture into the baking dish with melted butter.
Bake for 35 minutes. Let cool 45 minutes then cut into bite-size squares.
