Assemble one slider by spreading mayonnaise on the bottom of the roll.
Add a layer of turkey, then bacon (breaking it in half or thirds to fit on the roll), a few diced pimientos (or tomatoes), half a slice of Gruyere, and a sprinkling of Parmesan cheese. Replace the top. Repeat with remaining rolls to assemble a total of 12 sandwiches.
Arrange sandwiches in a single layer in a greased 9-inch-square baking pan.
In a small skillet, melt butter over medium heat. Add onion; cook and stir 1-2 minutes or until tender.
Whisk in the brown sugar, Worcestershire sauce, and garlic powder. Continue whisking until brown sugar is dissolved.
Pour butter sauce over sandwiches.
Cover with aluminum foil and refrigerate for several hours.
Preheat oven to 350°F (180°C). Bake, covered, 25 minutes.
Remove foil and bake uncovered for an additional 5 minutes or until golden brown.