Remove and discard the stems from the champignons. Peel the mushroom caps.
Grate the garlic clove. Mix with 3 tbsp soy sauce and some of the vegetable oil. Add 1 pinch salt if desired.
Place the chicken fillet in a bowl and pour roughly 1 tbsp of the marinade over it. Cover and refrigerate for at least 30 minutes.
Place the peeled mushroom caps in another bowl and pour the remaining marinade over them. Cover and refrigerate for at least 30 minutes.
Slice the onion bulb thinly into rings.
In a small bowl, combine the onion rings with 1 tbsp apple cider vinegar, 2 pinches sugar, 1 pinch salt, and roughly 50 ml of boiling water. Let sit for at least 10 minutes.
Remove the chicken and mushrooms from the refrigerator. Pat dry with paper towels, wiping away any garlic remnants.
Heat a frying pan over medium heat. Fry the marinated chicken fillet until cooked through (74°C / 165°F internal temperature), about 5–7 minutes. Transfer to a plate and slice thinly.
Brush all mushroom caps with the remaining vegetable oil. Sprinkle 2 g sesame seeds on 4 of the caps.
Place the mushroom caps on a baking tray. Bake at 200°C (392°F) for 10–12 minutes.
Slice the pickled cucumber thinly.
Slice the hard cheese.
Assemble each burger: place one mushroom cap gill-side up, add a lettuce leaf, sliced chicken, a slice of cheese, cucumber slices, and pickled onions, then cover with another mushroom cap gill-side down.
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