Preheat the oven to 350 degrees. Cut the onions into slices between ¼ inch and ½ inch thick. Try to get similar thickness on them, so that they cook evenly.

Into a large Stainless Steel Skillet over medium low heat, melt 2 tablespoons of butter. Toss in the onions and salt and break them up with a wooden spoon. Cover and cook for 4 minutes. Take off the cover and cook for an additional 10 to 12 minutes, stirring every few minutes. If the onions start to brown, turn down the heat.

While the onions are caramelizing, prepare the sauce. Into a small bowl, whisk together the mayonnaise, prepared horseradish and black pepper. Set aside.

After about 15 minutes, the caramelized onions should be complete. Place them into a bowl with the remaining butter in the pan.

Slice halfway between the Kings Hawaiian Rolls to create a top and bottom layer. Add all the roast beef on the bottom layer. Spread the Horseradish Aioli on the top half.

Next, add the caramelized onion, then the cheddar cheese over the roast beef.

Place the top half over the bottom half and bake on a cookie sheet for 13 minutes.

While the sliders are baking, heat a small skillet over medium low heat. Add in the remaining 2 tablespoons of butter and melt. Toss in the garlic, and cook for 1 to 2 minutes, stirring throughout to avoid burning. Transfer to a bowl.

Once the roast beef sliders come out of the oven, brush with garlic butter and top with fresh parsley. Cut with a large knife into individual sliders and enjoy!