Preheat oven to 350. Spray an 8 x 12 pan with cooking spray. Drain spinach in colander and squeeze out moisture. Combine cheese, spinach and onion in large bowl. Mix. Stir salt and baking powder into flour and mix with spinach. Mix eggs, butter, milk, and stir into the other ingredients. Pour into pan. Bake 30-35 minutes.
You can also make these in mini muffin tins as hors doeuvres. Makes about 42. Bake for 18-20 minutes.