Step 1: In a bowl, combine ground beef, salt, pepper, and garlic powder. Mix gently until just combined.
Step 2: Divide the mixture into 12 equal portions and shape into small patties, about 2 inches in diameter.
Step 3: Heat vegetable oil in a large skillet over medium-high heat. Cook patties for 2-3 minutes per side until browned and cooked through.
Step 4: Place a slice of cheese on each patty during the last minute of cooking to melt.
Step 5: While patties cook, mix mayonnaise, ketchup, pickle relish, and mustard in a small bowl to make the sauce.
Step 6: Lightly toast slider buns if desired. Spread sauce on both halves of each bun.
Step 7: Assemble burgers by placing a patty on each bottom bun, then adding onion slices and pickle slices. Top with bun halves.
Step 8: Serve immediately while warm.