Step 1

Heat oven to 350ºF.

Step 2

Bake butter in 9-inch square baking dish in 2 min. or until melted. Remove dish from oven; brush butter onto bottom and sides of dish.

Step 3

Beat eggs with mixer in large bowl. Add flour, milk and baking powder; mix until blended. Stir in spinach, cheeses and onion. Spoon into prepared dish.

Step 4

Bake 35 min. Let stand 15 min. before cutting into squares to serve.