Start by chopping the onions and sautéing them in a bit of butter over medium-low heat. Let them caramelize slowly, stirring occasionally, which should take about 10 to 15 minutes until they're soft and deeply golden.
While the onions cook, rub the sirloin steaks with garlic, then grill or fry them to your liking. Cook the bacon until crisp, and fry the eggs with a sprinkle of salt and pepper. Once the brioche buns are cool, slice them in half and toast them lightly.
To assemble, spread mustard on one half of each toasted bun. Layer with a sirloin steak, a slice of ham, a slice of mozzarella, a strip of bacon, a tomato slice, and some lettuce. Top with a fried egg, a generous sprinkle of chimichurri, and some of those caramelized onions, then serve immediately.