Preheat oven to 400°. Cut or separate top and bottom of slider buns. Place 12 bottom buns on separate sheet pan.
Spread the ground beef evenly out onto a 13 x 18 rimmed sheet pan, covering the entire pan. Sprinkle with salt and pepper. Place on the middle rack of oven and bake for 20 minutes.
Meanwhile, melt butter in a skillet over medium-high heat. Add sliced mushrooms to melted butter and saute until mushroom are tender. Set aside.
When ground beef is cooked, remove from oven and pour fat off the pan. *Ground beef will shrink so this is normal. Using a pizza wheel or sharp knife, cut the cooked beef into 12 even squares. Place one square onto each bun bottom on prepared pan. Top each beef square with 2 slices of swiss cheese. Place the pan back in the oven for a few minutes to melt the cheese. Remove pan from oven. Divide the cooked mushroom evenly on top of the melted cheese on sliders.
In a small bowl, combine the mayonnaise with Dijon mustard. Place a teaspoon of mayonnaise on each top bun and place on sliders. Return sliders to the oven for a few minutes to heat the top bun. Remove from the oven and serve hot