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