PREHEAT oven to 350° F. Grease 9-inch springform pan. Wrap foil around bottom of pan.
MELT butter in medium skillet over medium-high heat. Add onion; cook, stirring occasionally, for 3 to 4 minutes or until onion is tender. Place in large bowl. Add beans, evaporated milk, eggs, flour, Worcestershire sauce and black pepper; stir well. Stir in cheeses. Pour mixture into prepared pan.
BAKE for 35 to 40 minutes or until knife inserted in center comes out clean. Let cool for 10 minutes. Run knife around edge of frittata; remove side of springform pan. Serve warm with tomato sauce or white (béchamel) sauce.
*NOTE: To save time, 2 cans (14.5 oz. each) green beans, unheated and drained, can be substituted for the fresh or frozen beans.