Set your oven to 350°F so it’s ready when your sliders are assembled.
Dice the onion and add it to a large skillet over medium heat along with the ground beef, hamburger seasoning, and 1 tsp of garlic powder. Cook until the beef is no longer pink, breaking it up with a meat chopper or spoon as it cooks.
Slice the slider rolls in half horizontally and place the bottom halves into a casserole dish.
Start by laying 6 slices of Swiss cheese over the bottom rolls.
Spread the cooked beef mixture evenly over the cheese, then top with sliced mushrooms.
Add the remaining 6 slices of Swiss cheese on top of the mushrooms, and place the top halves of the rolls over everything.
Melt the butter and mix it with the remaining ½ tsp of garlic powder. Use a pastry brush to generously coat the tops of the rolls with the garlic butter mixture.
Cover the casserole dish with aluminum foil and bake for 15 minutes. Remove the foil and bake for an additional 3 minutes to let the tops get golden and slightly crispy.
Cut into 12 sliders and serve them warm. Watch as they disappear in minutes!