Slice the tomato and cucumber into thin rounds.
Take half a red onion and slice it into fine rings. Set them aside for later. Finely chop the other half of the red onion for the caramelized onions.
Slice the zucchini into about 0.5 cm thick rounds and the bell pepper into bite-sized pieces.
Prepare the secret sauce: In a small bowl, mix equal parts ketchup and mayonnaise with a splash of lemon juice, a splash of soy sauce, and half a teaspoon of spicy mustard. Stir everything well.
Slice the burger buns into three parts each: a bottom bun, a middle piece, and a top bun.
Divide the ground beef into four equal balls. Optionally, mix it with some pepper, salt, and a few old breadcrumbs if you like the patties a bit softer.