Start by cooking the bacon in a skillet over medium heat for 10-15 minutes until crispy, then drain on paper towels and chop into small pieces. While the bacon cooks, prepare the baconnaise by whisking together the mayonnaise, ketchup, mustard, Worcestershire sauce, apple cider vinegar, honey, dry rub, and red pepper flakes in a bowl. Reserve half of the chopped bacon to garnish the finished burgers and fold the other half into the baconnaise for maximum flavor distribution. In a separate bowl, combine the salt, black pepper, garlic powder, and smoked paprika—this seasoning blend will be applied during cooking for better crust development. Form the ground beef into 6 loosely packed balls (don't compress them too much or they'll be tough), then refrigerate for at least 30 minutes while you finish other prep work.

Slice the onions into thin 1/8-inch rings and place them in a colander or fine-mesh strainer. Squeeze out as much liquid as possible by pressing gently—this prevents the burgers from steaming and helps achieve a better crust. Set the drained onions aside on a plate. Split the brioche buns and set them aside, ready for toasting once the burgers are nearly finished cooking.

Heat a large cast-iron skillet or griddle over medium-high heat and add the avocado oil, letting it shimmer for about 1 minute until it just begins to smoke. Working quickly, place one beef ball on the hot surface and immediately top it with a generous handful of the prepared onions. Using a sturdy spatula, press down hard on the meat and onions for about 30 seconds to create a thin, crispy patty—this contact with the hot surface creates the prized flavorful crust. I like to let the onions caramelize slightly on the bottom while pressing, which adds a sweet, savory depth to the burger. Sprinkle the seasoning blend (from Step 1) evenly over the top of the patty, and cook for 3-4 minutes without moving it, allowing the crust to develop.

Once the bottom is deeply browned and crispy, flip the patty carefully using your spatula. Spread a thin layer of yellow mustard across the cooked top side (now facing up), then immediately top with one slice of pepper jack cheese. The residual heat will melt the cheese perfectly. At this point, place the brioche buns cut-side down on the griddle next to the burgers to toast for 30-60 seconds until lightly golden—they toast quickly, so watch them carefully. If needed, work in batches to manage space on your griddle without crowding.

Transfer the toasted buns to a clean work surface or serving platter. Spread a generous dollop of baconnaise on the bottom bun of each burger. Stack two of the finished patties (with melted cheese facing up) onto the sauce, then top with another dollop of baconnaise. I like to add the reserved crispy bacon pieces and any remaining caramelized onions from the griddle right at the end—they add crunch and keep everything from sliding around. Cap with the top bun and serve immediately while everything is still hot and the cheese is gooey.