Mix mayonnaise and mustard, season with pepper. Wash the cucumber and cut into very thin slices. Wash the parsley and remove the leaves. Remove the crusts from toast slices. Spread 4 slices each with mustard mayonnaise, cream cheese or horseradish.
Place roast beef and parsley on 2 slices of toast coated with mustard mayo, cucumber slices on 2 cream cheese toasts and smoked salmon on 2 horseradish toasts. Place the remaining slices of toast with the same coating on top and press down gently. Cut the sandwiches diagonally or lengthwise in half or crosswise into quarters.