Get yourself a large bowl and combine your ground beef, white onion, wholegrain mustard, sweet chili sauce, Worcestershire sauce, egg, and panko breadcrumbs. Season with salt and pepper.
Stir the mixture until you have a thoroughly combined burger patty mixture.
Form into 4-8 patties depending on how thick you like your hamburgers. I'd recommend making them as wide as your burger buns. Mine were around 4 inches/10 cm in diameter. You can then either make then into thin burger patties (around ½ inch thick), or thicker hamburger patties (around 1 inch thick). Use your hands to form them into circular patties, then set them aside while your pan heats.
Place a non-stick pan/skillet on medium heat and drizzle with a teaspoon of vegetable oil. Add in your hamburger patties and cook them for 3 minutes on each side for thin patties and 5 minutes on each side for thick patties. Then, reduce the heat, add on your cheese (optional), place the lid on the pan and cook for a further 2 minutes until the cheese starts to melt. Remove the patties from the pan and set aside ready to assemble.
Time to assemble your hamburgers. Begin by lightly toasting your bun, then adding sauce to the base, followed by lettuce, tomato, pickled red onion, your homemade hamburger patty, onion relish, and the top of your bun.
Press down so that all the juices combine, and enjoy with a side salad or homemade French fries.