Sprinkle the cucumbers with salt, draw juice in a sieve, squeeze well. For the horseradish foam, spread the avocado flesh through a sieve, mix with paste, cream cheese and heavy cream, season.
Cut away the crust of the toast, then slice lengthwise. Spread the slices of bread thinly with horseradish cream. Top each of two slices with cucumber and borage flowers, two slices with graved salmon, and one slice with salmon roe. Spread other side of toast slices with horseradish foam as well, layer alternately, wrap tightly in plastic wrap, weigh down with a board and set aside to cool. Set remaining paste aside to cool with lid closed.
Serve: Brush slices all over with remaining paste.
Cut into triangles or cubes, insert wooden sticks.