Mix mayonnaise, ketchup, and mustard in a small bowl.
Slice the tomato into 4 circles, the onion into 4 rings, and the pickled cucumber into slices.
Form the minced meat into 4 flat patties, each the width of a bun.
Heat sunflower oil in a frying pan over medium-high heat. Cook the patties for 3-4 minutes per side until the internal temperature reaches 63°C (145°F).
Top each hot patty with 2-3 slices of cheese to melt.
Cut the buns in half and lightly toast the insides in the frying pan over medium heat for 1-2 minutes until lightly golden.
Spread the sauce mixture on the inside of the top bun halves.
Assemble each burger: bottom bun, lettuce leaf, patty with melted cheese, tomato circle, onion ring, pickled cucumber slices, then top bun half.