An easy-to-put-together cold appetizer like this is key for cocktail parties, since all the elements can be made ahead, refrigerated, and then assembled at the last minute.
Place the smoked salmon, fromage blanc, onion, lemon zest and juice, capers, salt, and measured pepper in a food processor fitted with the blade attachment. Process, stopping to scrape down the sides of the bowl as necessary, until smooth, about 20 seconds. At this point, the salmon mixture can be transferred to a piping bag fitted with a 1/2-inch star tip and refrigerated for up to 4 hours.
When ready to serve, arrange the cucumber slices on a serving platter. Pipe about 2 teaspoons of the salmon mixture onto each slice. Top with the desired amount of caviar or salmon roe (you may have some left over) and serve immediately.
