Put the ingredients for the marinade into the saucepan (press garlic through the press) and mix thoroughly to combine. Cook on a very strong fire to reduce excess water for about 15-20 minutes. Cool the marinade to room temperature, coat cleaned chicken breasts with it, and refrigerate for at least 1 hour.
Combine mayonnaise, lemon juice, finely chopped parsley, and a pinch of pepper and salt. Refrigerate the sauce.
For frying pan version: Place marinated meat on a hot, greased pan. Fry for about 4 minutes on medium heat, turn and fry for another 5 minutes. Add a few tablespoons of water, cover with a lid, reduce heat, and stew for 1 minute. Transfer to a plate.
For barbecue version: Grill marinated fillets for approx. 15 minutes, turning occasionally. Once browned, let rest for 1-2 minutes before transferring to a plate.
Grill mushrooms cut into thick slices for about 1 minute on each side, sprinkle with salt and pepper. Toast slices of bread for about 1 minute on each side.
Spread sauce on a slice of toasted bread, add grilled mushrooms and chicken breast, top with another slice of bread spread with sauce. Press lightly and serve.
