01 - Heat butter and olive oil in a large skillet over medium-low heat. Add sliced onions and salt. Cook, stirring occasionally, for 20–25 minutes until onions are deeply golden and soft. Stir in sugar and balsamic vinegar for extra flavor, if desired. Remove from heat and set aside.
02 - In a bowl, gently mix ground beef with Worcestershire sauce, salt, and pepper. Do not overwork. Divide into 4 equal portions and shape into patties slightly larger than the buns.
03 - Heat a grill or skillet over medium-high heat. Cook patties for 3–4 minutes per side for medium, or to desired doneness. Top with cheese slices in the last minute of cooking, covering to melt.
04 - Toast the burger buns lightly on the grill or in a pan.
05 - Place lettuce and tomato on the bottom bun, add the burger patty, top generously with caramelized onions, pickles, and condiments. Finish with the top bun.
06 - Serve immediately with your favorite sides.