Cast-Iron Smashed Burgers with Onion

Slice 2 yellow onions into thin half-moons (3mm). Target: consistent thickness for even caramelization.

Portion 680g ground beef into 4 balls (~170g each). Do NOT overwork — loose packing preserves juiciness. Season with 8g salt and 2g black pepper, tossing gently.

Slice 1 tomato into 4 rounds. Wash and dry 4 lettuce leaves. Set toppings aside on a chilled plate.

Spread 4 bottom bread slices with 15ml ketchup each. Spread remaining 4 slices with 30ml mustard and 60ml mayo combined.

Heat a stainless skillet over medium-low (165°C target surface temp). Add 30g butter, let foam subside.

Add sliced onions, stir to coat. Cook 18-22 minutes, stirring every 3 minutes. Target: deep golden-brown, jammy texture. Remove when moisture has fully evaporated and edges are crispy-dark.

Place cast-iron skillet over high heat. Allow 3 minutes to reach smoking point. Lightly grease with butter.

Add beef balls to skillet, leaving 5cm spacing. Immediately smash with a flat spatula, applying 8-10kg of pressure for 30 seconds. Target thickness: 2cm.

Cook 90 seconds without disturbing. Observe edges — when 60% gray, flip once. Cook 45 seconds on reverse. Target internal temp: 65°C for medium.

Top each patty with 1 slice cheddar, cover loosely with foil, rest 90 seconds to melt cheese.

Place each patty on a ketchup-spreaded bread slice. Layer 1 lettuce leaf, 1 tomato round, and caramelized onions.

Cap with condiment-spreaded bread slice. Press gently with spatula for 10 seconds to set structure.

Serve immediately. If holding, tent loosely — condensation ruins crust.