Coat a 9 x 11 casserole dish with nonstick cooking spray.
Arrange bottom portions of slider rolls in the casserole dish.
Assemble sliders by adding a slice of turkey, then bacon crumbles, a few diced pimentos, half a slice of Gruyere cheese, and sprinkle with Parmesan cheese. Cover with the top halves of the rolls. Repeat the process for each sandwich.
In a small skillet, over medium heat, melt butter. Add diced onion, and sauté until tender and translucent, about five minutes.
Whisk in light brown sugar, Worcestershire sauce, and garlic powder to the cooked onions. Continue whisking until the brown sugar has dissolved.
Pour butter mixture over the sandwiches.
Cover with aluminum foil and refrigerate for several hours.
Preheat oven to 350°F
Bake with the aluminum foil still covering the casserole dish for 25 minutes.
Remove aluminum foil, and bake uncovered for an additional five minutes or until slider rolls are golden brown.