Heat a large skillet over medium-low heat and add 2 tablespoons of butter. Once melted, add thinly sliced Vidalia onions, stirring to coat. Cook for about 15 minutes, stirring occasionally, until the onions become soft and translucent.

Reduce the heat slightly and continue to cook the onions for 30–45 minutes, stirring every 5–10 minutes, until they turn a deep golden brown.

Pour a splash of dry sherry into the skillet to deglaze, scraping up any brown bits. Cook for an additional 2-3 minutes.

In a large bowl, combine ground beef, half of the caramelized onions, Worcestershire sauce, Dijon mustard, and garlic powder. Mix gently and form into four patties.

Preheat your grill to medium-high heat and cook burger patties for about 4-5 minutes on each side.

Assemble your burgers on warmed brioche buns and top with remaining caramelized onions, lettuce, and tomato.