Ingredients (2 servings):

Burger:
• 2 brioche burger buns
• 300–360 g ground beef (ideal: 20% fat content; 2×150–180 g)
• Salt, freshly ground pepper
• 1 tbsp high-smoke point oil (or a little beef tallow)
• 1-2 slices cheddar or gouda (good melting cheese)
• 1 tbsp butter for the buns

Caramelized onions:
• 2 large red onions (thinly sliced)
• 1–2 tbsp butter
• 1 tsp brown sugar (optional, but speeds up and deepens flavor)
• 1–2 tbsp balsamic vinegar or apple cider vinegar
• Salt, pepper

Quick burger sauce (optional, but recommended):
• 3 tbsp mayonnaise
• 1 tsp mustard
• 1 tsp ketchup
• 1–2 tsp finely chopped pickles
• 1 pinch smoked paprika or chili (optional)

Preparation:

Caramelized onions (25–35 minutes)
Start the onions in butter over medium heat with a pinch of salt. After 10 minutes, when they collapse and turn translucent, lower the heat and caramelize to a deep golden brown, stirring occasionally. At the end, add sugar (if using), then the vinegar, and cook off the acidity for 1–2 minutes. Pepper, and salt to taste.

Sauce (2 minutes)
Mix the mayonnaise, mustard, ketchup, and add-ins together. Set aside.

Shaping patties
Divide the ground meat into 2 parts, shape loosely (don't overknead). Make a slight depression in the center (so it doesn't dome). Salt and pepper immediately before frying.

Frying
Fry the patties in a smoking hot pan (cast iron is ideal) with a little oil: for 150–180 g patties, usually 2–3 minutes per side over high heat to crust. After flipping, place the cheese on top and melt under a lid for 30–60 seconds. Remove at desired internal temperature and rest for 3–5 minutes.

Toasting buns
Toast cut-side down in butter until golden brown. This adds flavor and "insulates" the buns against the juices. Assembly Bottom bun + sauce, then the cheesy patty, followed by plenty of caramelized onions. Close with the top bun. Serve immediately.

💡 Pro tip :

The secret to a "picture-perfect", stably high-buildable burger is the two-step texture:
Caramelize onions until jammy thick (don't let them remain watery), otherwise the tower will slide.
Melt the cheese under a lid for 30–60 seconds: this way it completely tightens onto the meat and acts as a natural "glue" holding the onion and patty together.

If you like, I can add an "extra juicy, but still biteable" restaurant assembly order (sauce ratios + exact onion consistency), as well as variations (with bacon, mushrooms, spicy honey glaze).