Cut the rind off the toast. Roll out the bread slices nice and flat with a rolling pin.
Put the softened butter in a bowl. Mix half of the cress leaves into the butter. Brush the toast slices with the cress butter.
Roughly chop the trout fillets. Puree in a bowl with the crème fraîche, lemon juice, mustard, salt and pepper with an electric cutting stick.
Spread this cream on the toast slices. Roll up the slices, cut diagonally in the middle. Place 4 rolls on each plate and sprinkle with the rest of the cress.