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.
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 and breadcrumbs into egg mixture; spoon into a lightly greased 6-qt. slow cooker.
Cover and cook on LOW 4 to 5 hours or until center is set and edges are golden brown.
Note: Onions can be cooked up to 2 days ahead. Cover and chill until ready to assemble pudding.