01 -
Season the sirloin with kosher salt and black pepper. Let it rest at room temperature for about 30 minutes before cooking to allow the meat to come to temperature and absorb the seasonings.
02 -
While the steak is resting, prepare the caramelized onions. Melt 2 tablespoons of butter in a large non-stick skillet over medium-low heat. Add the sliced onions and cook for 25-30 minutes, stirring occasionally, until they become very tender and turn a rich golden brown color. Transfer to a plate and set aside.
03 -
Preheat the oven to 350°F. Spray a 9x13-inch baking dish with nonstick cooking spray and set aside.
04 -
Using the same skillet, melt the remaining tablespoon of butter over medium-high heat. Add the steak and cook for 4-5 minutes on each side, or until it reaches your preferred level of doneness. Tilt the skillet and spoon the melted butter over the steak as it cooks. Set the steak aside to rest for about 5 minutes, then slice it against the grain as thinly as possible.
05 -
Split the brioche rolls horizontally and place the bottom halves into the prepared baking dish. Pour about ¼ cup of the melted cowboy butter evenly over the bottom halves. Layer with the sliced steak, jalapeño slices, shredded cheddar cheese, and caramelized onions. Place the top halves of the rolls on top and brush with another ¼ cup of melted cowboy butter.
06 -
Cover the baking dish with aluminum foil and bake for 15 minutes, or until the cheese is hot and bubbly. Remove the foil and continue baking for an additional 3-5 minutes until the tops of the sliders turn golden brown.
07 -
Remove the sliders from the oven and brush the tops with the remaining melted cowboy butter while they're still warm. Slice between the rolls to separate and serve immediately.