Melt butter in a large skillet over medium heat; add onion, and sauté 20 minutes or until caramel colored. Stir in balsamic vinegar, salt, and 1/8 tsp. pepper.
Preheat oven to 350°. Prepare pasta according to package directions.
Meanwhile, stir together chicken, 1/2 cup Parmesan cheese, and next 6 ingredients; stir in pasta and caramelized onion. Spoon mixture into 6 lightly greased 8-oz. ramekins or a lightly greased 11- x 7-inch baking dish. Sprinkle with almonds and remaining 1/2 cup Parmesan cheese.
Bake at 350° for 35 minutes or until bubbly.