Preheat oven to 350 F. Lightly spray a 9 x 13-inch baking dish with cooking spray.
Heat skillet over medium heat, adding butter & oil. Sauté sliced onions in butter & oil until soft, brown & slightly caramelized, reduce heat to low if necessary. Stirring occasionally, about 15-30 minutes.
In the meantime, prepare the sliders. Slice the “loaf” of brioche rolls in half lengthwise, with the bottoms slightly thicker than the tops, do not separate individual rolls.
Layer roast beef to slider bottoms, then layer cheese over the top of the roast beef, finishing with caramelized/sautéed onions. Cover with tops of bread rolls.
Drizzle
In a small saucepan, melt butter over medium heat; add mustard, brown sugar, Worcestershire sauce, garlic & onion powder, & dried thyme, stirring until the drizzle is warmed & the sugar has dissolved. Just a minute or two.
Baking
Evenly pour drizzle over the tops of the brioche rolls. Cover with sprayed aluminum foil & bake in a preheated oven for about 20 minutes. Remove foil & bake for 7-10 minutes until tops are lightly browned. Serve immediately.