Choose 2 medium-sized cremini mushrooms per serving and remove the stems (ingredient weight yield after stem removal). Slice each mushroom cap in half like burger buns. Combine all the sliced mushroom caps in a mixing bowl with salt, pepper, garlic, and olive oil. Wrap and set the bowl in your refrigerator to marinate at least 30 minutes (up to 60).
Preheat an oven to 375 degrees. In a new mixing bowl, combine the ground beef, salt, pepper, onion powder, and paprika. Form 2 burger patties per serving by first forming a 1 ½ – 2 inch meatball and then flattening into a patty. Arrange the mini burgers on a baking rack set over a sheet tray to catch drippings.
Bake the burgers for 15 minutes for medium-well done burgers, flipping them over after the first 10 minutes. When you flip the burgers over, lay ¼-slice pieces of swiss cheese over each patty before cooking the remaining 5 minutes.
After the burgers are cooled enough to handle, place each patty on the bottom piece of a mushroom cap. Top each burger off with the remaining caps and secure with toothpicks.