Preheat the oven to 350 degrees. Line a baking pan with foil and spray with cooking spray.
Use a serrated knife to slice through the middle of the Hawaiian rolls and open them up as one unit.
Mix together the mayonnaise, horseradish sauce, and lemon juice. Taste and season as needed, adding more horseradish sauce or lemon if you prefer.
Spread the mayonnaise mixture over both sides of the sliders. Top with the roast beef, followed by slices of cheddar cheese. Sprinkle with French fried onions.
Sandwich with the top layer of the slider buns. Whisk together the melted butter and garlic clove then brush the mixture evenly over top of the buns.
Cover the sliders with aluminum foil and bake for 15 minutes. Remove the foil and bake another 6-9 minutes, or until cheese is melted and buns are golden brown on top.