For the toadstools, remove crusts from toast and spread with room-temperature butter.
Cover four slices of toast with one slice of cheese each, season with salt and pepper.
Top four slices of bread with one slice each of the seltzer roll. Mix freshly grated horseradish with a squeeze of lemon juice and spread on the smoked meat.
Place smoked meat sandwiches on top of cheese sandwiches and cover each with bread. Spread cream cheese on top. Finely chop chives and apply enough of them to cover almost all of the cream cheese.Cut bread towers into four pieces each.
Cut tomatoes so flat that they remain standing. Place a tomato on each bread tower and use mayonnaise to apply white dabs – like a toadstool. If the dabs slip off the tomato, simply cut tomato in half and apply dabs to cut side.
Arrange the toadstools decoratively and serve.