01 - In a large skillet over medium heat, melt butter with olive oil. Add sliced onions and salt. Cook, stirring occasionally, for 15 to 20 minutes until soft and golden. Add sugar to enhance sweetness. Transfer to a bowl and set aside.
02 - Preheat a grill or skillet to medium-high heat.
03 - In a bowl, gently mix ground beef with salt, pepper, Worcestershire sauce, and Dijon mustard. Form into 8 equal small patties, slightly larger than the buns as they will shrink during cooking.
04 - Cook the patties for 2 to 3 minutes per side for medium doneness. If using cheese, add a slice to each patty in the last minute of cooking and cover to melt.
05 - Lightly toast the slider buns on the grill or in a skillet if desired.
06 - Spread mayonnaise or aioli on the bottom half of each bun. Top with lettuce, a beef patty, a generous spoonful of caramelized onions, and pickles. Cover with bun tops and serve immediately.