Combine ground beef, onion soup mix, salt, and pepper. Rest for flavor melding. Shape into four equal patties, pressing an indentation in the center to prevent puffing.

Melt butter in a skillet. Add onions and thyme, cook until soft. Add water, cover, and simmer until mostly evaporated. Uncover and cook low-medium, stirring often, until deeply golden brown (about 20-30 minutes).

Add brandy or sherry to the caramelized onions and stir to scrape up any browned bits from the pan. Remove from heat.

Grill or pan-fry patties over medium-high heat for 3-4 minutes per side.

Top each burger with caramelized onions and cheese slices. Lower heat, cover, and cook for 2-4 minutes until cheese is perfectly melted and gooey.

Lightly butter the cut sides of your buns and toast them on the grill or in a skillet until golden brown.

Place a patty on the bottom bun, pile on the sweet caramelized onions, and top with the other half of the bun.