In a bowl, combine 500g minced meat, 300g round beef, 1 egg, and lightly salt and pepper. Mix gently until just combined. Shape into 6 equal balls, then flatten each into a patty using a plate. Refrigerate until ready to cook.
Heat some oil in a large pan over medium-high heat. Fry burger patties for 3-4 minutes per side until browned. Transfer to a plate.
In the same pan, fry 12 slices bacon for 5-7 minutes until crispy. Transfer to paper towels to drain.
Slice 3 leaves lettuce into strips. Slice 1 onion into rings. Slice 2 tomatoes into slices.
In a bowl, mix 200g mayonnaise, 2-4 tablespoons ketchup, and 2-3 pinches BBQ spice mix. Add a small amount of liquid smoke if desired. Stir in 2-3 tablespoons of bacon grease from the pan. Add a small amount of sugar if the sauce tastes too spicy.
Heat a second pan over medium heat. Place 6 burger bun halves cut-side down and toast for 1-2 minutes until lightly browned, without using oil or fat.
Place 1 slice Emmentaler cheese on each cooked burger patty. Return patties to the first pan over medium heat for 2-3 minutes until cheese begins to soften but does not melt completely.
Assemble each burger: On the bottom bun half, layer 2 slices Emmentaler, 2 slices bacon, 1 cheese-topped patty, lettuce strips, onion slices, and 2 tomato slices. Top with 1-2 tablespoons sauce and the top bun half.