Gather the ingredients for the burgers and caramelized onions.
Mix the ground beef, mustard, olive oil, salt, and pepper with a fork.
Divide the mixture into six patties, each about ⅓ pound.
Chill the patties in the refrigerator for a few hours.
Slice the yellow onions and caramelize them in a pot with thyme, butter, and olive oil.
Cook the onions for about 30 minutes until translucent and caramelized, then add vinegar, salt, and pepper.
Grill the burger patties for about 5 minutes on each side.
Toast the English muffins on the grill.
Dress the burgers with mayonnaise and top with caramelized onions before serving.