Heat a medium skillet over medium-low. Add butter and olive oil. Add the thinly sliced onions and a pinch of salt; cook, stirring frequently, until soft and lightly golden, about 12–15 minutes.

Finish jam

Increase heat to medium. Add granulated sugar and continue to cook 2–3 minutes to deepen color. Deglaze with balsamic vinegar and beef broth (or water), scraping any browned bits. Add thyme, salt, and pepper. Simmer until liquid reduces and onions are jammy, 3–5 minutes. Remove from heat and keep warm.

Garlic Aioli

In a small bowl, combine mayonnaise, grated garlic, lemon juice, Dijon mustard, salt, and pepper. Whisk until smooth. Taste and adjust lemon or salt. Refrigerate until assembly.

Sautéed Spinach & Mushrooms

Heat a skillet over medium-high heat with 1 teaspoon olive oil. Add sliced mushrooms and sauté until they release their liquid and begin to brown, about 4–5 minutes. Add minced garlic and cook 30 seconds until fragrant. Add baby spinach and toss until wilted, about 1–2 minutes. Season with salt and pepper. Transfer to a bowl and keep warm.

Burger Patties

Form two equal patties from 1 lb ground beef, handling lightly; make each roughly 4–5 inches wide and about 3/4-inch thick. Season both sides with kosher salt, black pepper, garlic powder, and brush or drizzle Worcestershire sauce on one side before flipping.

Preheat a heavy skillet or grill over medium-high heat and add 1 tablespoon olive oil (if using a skillet). When hot, place patties seasoned-side down. Cook without moving 3–4 minutes until a dark crust forms. Flip and cook 2–3 minutes for medium-rare to medium depending on thickness.

After flipping and when patties are almost to desired doneness (about 1 minute before), place one slice Swiss and one slice provolone on each patty. Cover the skillet or close the grill lid to melt the cheese, about 1–2 minutes.

Buns & Toasting

Butter the cut sides of the cheddar buns. In a separate pan or after patties finish, toast the buns buttered-side down over medium heat until golden and crisp, about 1–2 minutes. Keep warm.

Assembly & Serving

Spread about 1 tablespoon garlic aioli on the bottom half of each toasted cheddar bun.

Place the cheese-melted beef patty on the aioli. Top each patty with 1/2 of the sautéed spinach and mushrooms mixture (about 1/4 cup) and then 2–3 tablespoons onion jam.

Add optional pickles or red onion slices if desired. Spread a thin layer of aioli on the top bun, place on burger, and serve immediately.