1. Melt 4 Tbsp. butter in a large skillet over medium heat; add onions, and sauté 20 minutes or until golden brown. Place half of cooked onions in a lightly greased 8-inch square baking dish. Sprinkle evenly with half of cheese and 1/4 cup cracker crumbs. Top with remaining onions and cheese.
2. Whisk together eggs and next 3 ingredients in a medium bowl; pour over onion mixture.
3. Melt remaining 2 Tbsp. butter in skillet over medium heat; add remaining 1/2 cup cracker crumbs, and cook, stirring often, until crumbs are lightly browned. Sprinkle crumbs evenly over mixture in dish.
4. Bake at 350° for 20 minutes or until lightly browned and set.