Sweet Onion and Parsley Sandwiches
Place the bread slices in a single layer on work surface. Combine the mayonnaise and parsley in a small dish. Spread 1 tablespoon of the mayonnaise mixture on each slice. Pull a sharp swivel-blade vegetable peeler across the onion to make paper-thin slices. Arrange the onion in a single layer on 4 slices. Sprinkle generously with pepper. Close the sandwiches. Wrap each airtight in plastic. Refrigerate from 2 to 6 hours.
To serve, unwrap the sandwiches, then cut crosswise into 4 triangles. Arrange them on a platter; garnish with parsley sprigs.