Stir together first 3 ingredients in a large bowl. Combine flour and next 3 ingredients; gradually stir into egg mixture. Set aside.
Melt butter in a large skillet over medium heat; add onion, and cook, stirring often, 30 to 40 minutes or until onion is caramel colored. Remove onion from heat.
Stir onion into egg mixture; spoon into a lightly greased 13- x 9-inch baking dish.
Bake at 350° for 30 minutes or until set.