Drain tuna well in a sieve. Peel onions and cut them into thin rings. Clean and clean the mushrooms and cut them into slices. Season sour cream with salt and pepper.
Spread 1 tsp. of sour cream on each slice of toast. Cover 4 slices of toast with onion rings, tuna, 1 tsp. sour cream and mushroom slices. Finish with one slice of toast each. Cut off the edges, cut the slices diagonally in half.