Preheat oven to 350 F. Lightly spray a 9x13-inch baking dish with non-stick cooking spray.
Slice the buns open and lay the bottom part of the buns into the prepared baking dish.
In a small mixing bowl, whisk together the mayo and French onion soup mix. Spread onto the bottom bun layer, covering it completely.
Lay the cheese slices and roast beef onto the sliders next. Cut them or layer them as needed to fit well into the dish. Top with the top half of the slider buns.
Melt the butter and mix in the garlic and grated Parmesan cheese. Spread generously onto the tops of each slider. Bake in the preheated oven for 15-20 minutes.
Once baked, cut between the buns and then pull sliders out to serve for dinner. Serve with your favorite fruit, veggies, and chips! Enjoy!