Cut 18 rounds from cucumber (about 1/4 inch thick) and save any leftover for another use. Top each round with a thin slice of salmon cut to fit. Drizzle a little lemon juice over the salmon, then season with salt and pepper.
Place avocado, sour cream, 1 teaspoon pf lemon juice, salt, pepper, and cayenne pepper in a food processor and process until smooth and blended.
Garnish each cucumber slice with a dollop of avocado cream and sprinkle with green onions.