Step 2: place mince, onion, garlic, gherkin, tomato sauce and egg in a large bowl. Season with salt and pepper. Mix to combine. Roll 2 heaped tablespoons of mixture to make 8 balls. Flatten slightly. Place on a large plate. Cover with plastic wrap. Refrigerate for 30 minutes.
Step 4: heat a barbecue plate on medium-high heat. Cook patties for 3 to 4 minutes each side or until cooked through.
Step 6: meanwhile, preheat grill on medium. Cut each bun in half horizontally. Place halves, cut-side up, on a baking tray. Place patties on bottom half of buns. Top with one piece of cheese. Cook for 2 to 3 minutes or until buns are golden and cheese has melted.
Step 8: top mince patties with tomato sauce and american mustard. Sandwich with bun tops. Serve.