Separate toasties carefully. Toast one after the other. Pour the tuna into a sieve, drain well. Then crumble. Peel onions and cut into rings. Clean the mushrooms, (possibly wash them) and cut them in half.
Drain the mozzarella and cut into slices. Clean, wash and halve the tomatoes. Place toasties side by side on a baking tray lined with baking paper. Spread the sauce evenly on top.
Spread the tuna and onion rings over 4 toasties. Spread mushrooms and salami on 4 toasties and sprinkle with cheese. Cover the remaining 4 toasties with mozzarella and tomato. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 12-15 minutes.
In the meantime wash parsley, dab dry and chop finely. Remove the toasties from the oven and arrange them on a plate sprinkled with parsley or pepper. Serve garnished with basil as desired.