You can make the cream cheese mixture up to 2 days ahead.
Pulse first 3 ingredients in a food processor until smooth. Add watercress and next 3 ingredients; process until finely chopped.
Spread 1 Tbsp. cream cheese mixture onto each bread slice. Top with assorted vegetable slices and herbs in a single layer, pressing lightly. Trim crusts from bread. Cut each bread slice into 4 squares or 2 rectangles.
Try These Twists:
Goat Cheese Canapés: Omit watercress and parsley. Reduce cream cheese to 4 oz. Add 1 (4-oz.) goat cheese log, softened, with cream cheese and 2 tsp. chopped fresh thyme with salt in Step Top with: candy-cane beets and chives, or sliced squash and salmon caviar.
Bacon-and-Olive Canapés: Omit salt. Substitute 1/2 cup green olives, chopped, and 6 cooked and crumbled bacon slices for watercress and parsley. Add salt to taste. Top with: sliced grape tomatoes and hard-cooked eggs, or basil leaves, celery, and radishes.