Set your oven to 350°F (175°C) and allow it to preheat while you prepare the sliders. Dice the yellow onion into small pieces.

In a large skillet over medium heat, combine the ground beef, diced onion, hamburger seasoning, and 1 teaspoon of garlic powder. Cook the mixture, breaking up the meat as it browns, until it is no longer pink. This should take approximately 8-10 minutes. Drain any excess grease, if necessary, and set the beef mixture aside.

Carefully slice the slider rolls in half horizontally to create a top and bottom layer. Place the bottom half of the rolls into a casserole dish.

Lay 6 slices of Swiss cheese evenly over the bottom layer of the rolls. The cheese acts as a barrier to keep the bread from getting soggy and adds that signature creamy flavor.

Evenly distribute the cooked beef mixture over the layer of Swiss cheese. Spread it out to cover all the rolls. Top the beef with sliced mushrooms, spreading them evenly for a consistent flavor in every bite.

Place the remaining 6 slices of Swiss cheese on top of the beef and mushrooms. Then, gently place the top half of the slider rolls over the cheese to complete the assembly.

In a small microwave-safe bowl, melt the butter. Stir in the remaining ½ teaspoon of garlic powder until well combined. Using a pastry brush, generously coat the tops of the slider rolls with the garlic butter mixture, ensuring every roll is evenly covered.

Cover the casserole dish with aluminum foil to keep the rolls soft while the cheese melts. Transfer the dish to the preheated oven and bake for 15 minutes. Remove the foil and bake for an additional 3-5 minutes, or until the tops of the rolls are lightly golden and slightly crisp. Keep an eye on them to prevent over-browning.

Once baked, remove the sliders from the oven and let them cool for a minute or two. Cut them into 12 individual sliders. Serve and enjoy!