Start by dividing the ground beef into eight equal portions. Roll them into small balls and gently flatten each into a patty shape, about the size of your slider buns. Sprinkle both sides with a little salt and pepper. Let them rest while you prep the mushrooms.
Heat a tablespoon of butter in a skillet over medium heat. Add sliced mushrooms and stir occasionally until golden brown and tender—about 5-7 minutes. Season lightly with salt and pepper. Set aside when done.
Heat your grill or skillet to medium heat. Cook each patty for about 3-4 minutes on each side. Try not to press them down while cooking—this keeps the juices inside. Right after flipping, pile on the sautéed mushrooms and top with a slice of Swiss cheese. Cover the pan briefly to melt the cheese faster.
Melt the remaining butter in a clean skillet or directly on the grill. Place the inside of each bun face down until toasted golden and slightly crispy—this adds extra texture and flavor.
Start with the bottom bun, then place the cheesy mushroom patty on top. Add any extras you love (like onions or sauces), then crown it with the top bun. Serve warm while the cheese is still gooey and delightful.