In skillet cook bacon over medium heat until browned and crisp. Remove; drain on paper towels, reserving drippings. Crumble bacon; set aside.
In a large bowl whisk eggs with salt, and pepper. Stir in green bean mixture, bacon, and half of the cheese. Pour into skillet with bacon drippings; cook over medium heat. As mixture sets, run a spatula around edge of skillet, lifting egg mixture so uncooked portion flows underneath. Continue cooking and lifting until almost set. Top with remaining cheese. Cover; set aside.
Meanwhile, in a medium saucepan heat 1/2 inch oil over medium-high heat. Dip onion slices in milk, letting excess drip off. Toss slices in flour; shake off excess. Cook coated onion slices in batches in hot oil about 3 minutes or until golden and slightly crisp. Using a slotted spoon, transfer onion slices to paper towels to drain. Top frittata with fried onion.