1. 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.
2. Preheat oven to 350°. Whisk together eggs, cream, and Parmesan cheese in a large bowl. Combine flour and next 3 ingredients in a separate bowl; gradually whisk into egg mixture until blended. Stir onions into egg mixture; spoon into a lightly greased 13- x 9-inch baking dish.
3. Bake at 350° for 35 to 40 minutes or until set. Garnish, if desired.