Preheat oven to 350°. Without separating rolls, cut in half horizontally; arrange bottom halves in a greased 11x7-in. baking pan. Spread mayonnaise evenly across the bottom halves. Top each with turkey, bacon, pimientos, Gruyere and Parmesan cheese. Replace top halves of rolls.
In a small skillet, melt butter over medium heat. Add onion; cook and stir until tender, 1-2 minutes. Whisk in brown sugar, Worcestershire sauce and garlic powder. Cook and stir until sugar is dissolved; drizzle over sandwiches.
Cover and bake 25 minutes. Uncover; bake until golden brown, 5-10 minutes longer.
