Cut the tuna and cleaned mushrooms into 5-mm squares. Chop the onions and Italian parsley. Cut lemon into wedges.
Place all ingredients in (A) in a bowl and mix.
Make a round mold with a diameter of 10 cm and a height of 2 cm using aluminum foil. (Or a 10-cm ring mold)
Place the mold on the plate and add the contents of the bowl and flatten. When the shape is a thick patty, slowly remove the mold. Sprinkle with olive oil and black pepper to taste and garnish with watercress and lemon.
Rub the garlic onto the sliced baguette to serve with the patty, squeeze some lemon juice on the patty and enjoy with the baguette. After adding garlic, you can also enjoy the baguette lightly toasted.