Start to finish: 30 minutes
2 tablespoons butter
1 tablespoon olive oil
3 yellow onions, thinly sliced
1 tablespoon sugar
1 pound 85/15 ground beef
1 tablespoon Worcestershire sauce
1 teaspoon garlic salt
1 teaspoon pepper
4 ounces sharp cheddar, cut into 8 cubes
¼ cup brown mustard
¼ cup mayonnaise
8 mini pretzel buns
8 red lettuce leaves
Melt butter and olive oil together over medium-low heat in a large skillet. Add in onions and toss to coat. Sauté, stirring frequently until translucent. Reduce heat to low, stir in sugar and continue to cook until completely caramelized, a deep golden color, about 15 minutes.
While onions cook, preheat grill to medium-high heat.
In a large bowl, combine beef, Worcestershire, garlic salt and pepper. Form mixture into 8 small patties by flattening into a disc, adding a cube of cheddar to the middle, folding the edges into the middle and flattening again into a patty that’s equal in thickness on the edges and in the middle.
Grill 4–5 minutes before flipping and grilling an additional 3–4 minutes, until juices run clear and cheddar starts to melt out of the patty.
In a small bowl, mix together mustard and mayo.
In final minutes of grilling, add buns, cut-side down, to the grill to warm.
Remove patties and buns from the grill.
To assemble: Add one tablespoon of the mustard-mayo sauce to the bottom bun, top with lettuce, patty, caramelized onions, pickles and top bun.