In a medium bowl gently combine (don't over-mix) the minced beef, finely chopped red onion, ketchup, mustard, soy sauce, and black pepper. Use cold meat from the fridge, don't warm it up to room temperature. Form your meat into four, same size patties. Once shaped, season them with salt and grill on a hot frying pan (medium heat) for about 3-5 min each side.

In the meantime, cut the rolls in half and grill/bake them, until slightly crispy. At this point, you can place a slice of cheese on the flatter roll part, so it'll melt. If you prefer you can also place a slice of cheese on the hot burger.

Assemble yours burgers using all the additional ingredients and serve.