Anders Bandt

• 3/4 cup mayonnaise , preferably whole egg
• 1 large garlic clove , minced
• 3 large onions , peeled and finely sliced (brown, white or yellow) (see note)
• 40 g / 3 tbsp butter
• 2 tablespoon brown sugar
• 1 tablespoon balsamic vinegar
• 1/4 teaspoon salt & pepper
• 600 - 700 g / 1.2 - 1.4 lb grilling beef steak
• Salt and pepper
• 1 tablespoon vegetable oil
• 60 g / 2 oz rocket /arugula or other lettuce of choice
• Mustard (I use American)
• Some good ass bread

• Mix mayonnaise and garlic together. Set aside for 30 minutes for the flavor to develop

• Melt butter in a skillet over medium low heat. Add onions and stir to coat in butter. Place lid on (or place a baking tray over skillet) and leave for 20 minutes, stirring once or twice. Remove lid then cook for a further 20 minutes, stirring every now and then, until onion is golden (increase heat slightly if the color is not changing). Add sugar, vinegar, salt and pepper. Cook for a further 10 minutes until jammy. Remove from heat and keep warm.

• Take beef out of fridge 20 minutes before cooking. Season generously with salt and pepper.
• Heat oil in a skillet over high heat (or heat BBQ). Cook steak to your liking - if using a secondary cut like Bavette, skirt or flat iron, it's best medium rare.
• Transfer beef to a plate, cover loosely with foil and rest for 5 - 10 minutes. Then slice thinly against the grain.
• Cut your good ass bread and toast lightly.
• Spread the bottom piece generously with garlic aioli. Top with rocket, then tomato slices, beef, caramelized onion. Spread the top piece of bread with mustard then place on top.

3 onions seems like a lot but properly carmalized onions reduce down a lot

Context here