Heat a wide skillet over medium heat. Add 1 tablespoon olive oil and 2 tablespoons butter. When butter melts and foam subsides, add the thinly sliced onions and stir to coat.

Cook onions slowly, stirring every 3–5 minutes. After about 10 minutes they will soften; lower heat to medium-low if they begin to brown too quickly. Continue cooking for 20–30 minutes, allowing sugars to develop and onions to take on a deep golden color.

When onions are deeply golden (about 25–30 minutes total), sprinkle in 1/2 teaspoon granulated sugar and stir to help with caramelization. Add 1 tablespoon balsamic vinegar and 1/2 teaspoon Worcestershire sauce and stir, scraping up browned bits from the pan.

Pour in 1/4 cup beef stock and add 1 teaspoon fresh thyme, 1/4 teaspoon salt and 1/8 teaspoon black pepper. Simmer on low until most liquid reduces and mixture is thick and jammy, about 4–6 minutes. Taste and adjust seasoning. Remove from heat and keep warm.

Sliders

Form patties

Divide 1/2 lb ground wagyu into two equal portions (about 4 ounces each). Lightly form into thin patties slightly larger than the buns (they will shrink). Do not overwork the meat. Season both sides with 1/2 teaspoon fine sea salt and 1/4 teaspoon freshly ground black pepper.

Cook patties

Heat a cast-iron skillet or heavy pan over medium-high heat until very hot. Add 1 teaspoon olive oil and 1 tablespoon unsalted butter; when butter is foaming, add the patties. Cook without moving for 2–3 minutes to get a good sear. Flip and cook another 1.5–2 minutes for medium-rare to medium (adjust time for desired doneness).

About 30–45 seconds before the patties are done, top each patty with the sliced brie (1 ounce per slider) so it melts. Remove patties to a plate and tent loosely with foil to rest for 2–3 minutes.

Toast buns

Slice the brioche slider buns and butter the cut sides with 1 tablespoon unsalted butter divided. Return skillet to medium heat and toast cut sides until golden, about 45–60 seconds.

Spread 1/2 teaspoon Dijon mustard and 1/2 teaspoon mayonnaise on the bottom halves of each toasted bun.

Divide the beefed-up caramelized onions between the two sliders (about 2–3 tablespoons each) and place a brie-topped wagyu patty on top. Add microgreens or arugula if using, then crown with bun tops.

Finish & Serve

Serve the two onion sliders immediately while warm. Optional: serve with extra caramelized onions on the side or a small pot of extra Dijon-mayo for dipping.