Melt the unsalted butter in a large skillet over medium heat. Add the sliced onions and a pinch of salt. Cook slowly, stirring occasionally, until the onions become a deep golden brown and caramelized, about 20-25 minutes.
While the onions are caramelizing, take your ground sirloin and divide it into 2.5-ounce portions. Gently form each portion into a patty, slightly larger than the slider buns. Season with kosher salt and pepper.
Preheat your grill or stovetop skillet over medium-high heat. Add the patties and cook for about 3-4 minutes on each side, until desired doneness.
In the final minute of cooking, place a slice of provolone cheese on each patty and cover to help the cheese melt.
Spread rosemary aioli on the bottom half of each slider bun. Top with a patty, caramelized onions, and the top half of the bun.
Arrange the assembled sliders on a serving platter. Serve warm or at room temperature with extra rosemary aioli on the side.
