Recipe(tried): Greek Spinach Squares

This is the result of combining and tweaking a few different recipes. It's good hot or just warm.

Greek Spinach Squares

4 Tbsp unsalted butter (melted)
1 cup milk
3 eggs
2 10-oz pkgs frozen, chopped spinach, thawed and squeezed dry
1 cup flour
1 tsp baking powder
1/2 tsp salt
8 oz grated Monterey Jack cheese
4 oz grated cheddar cheese
4 oz finely crumbled feta

Mix together the butter, milk, eggs, and spinach. Sift together and add the flour, baking powder, and salt. Stir in the cheeses. Pour into a greased and floured 13x9-inch pan. Bake at 350 degrees F for 35 minutes. Cool for about 30 minutes, cut into squares, and serve warm