01 - In a large bowl, gently mix ground beef, grated onion, minced garlic, Worcestershire sauce, Dijon mustard, salt, and black pepper until just combined.
02 - Divide the mixture into 8 equal portions and shape each into a small, slightly flattened patty.
03 - Heat a grill pan or skillet over medium-high heat.
04 - Cook patties for 2 to 3 minutes per side until desired doneness. Place a slice of cheddar cheese on each patty during the last minute to melt.
05 - In a separate skillet, melt butter over medium heat. Slice buns in half and toast the cut sides until golden brown.
06 - Spread mayonnaise and ketchup on bottom bun halves. Layer with lettuce, a beef patty with melted cheese, tomato slice, pickle slice, and cover with top bun halves.
07 - Serve sliders immediately while warm.