Begin by cutting the bread into circles with a biscuit cutter or cookie cutter. Combine the grated onion, mayonnaise, and black pepper. Spread mixture onto the cut out bread circles. Cover and refrigerate.
To prepare the tomatoes, peel, slice, and drain thoroughly between layers of paper towels. Sprinkle the tomatoes with seasoning salt before assembling the sandwiches. Place a tomato slice on top of the whole wheat bread. Top with parsley or basil. Place the white bread on top of each stack. Prepare the sandwiches several hours before the party. Cover with a damp towel and refrigerate until ready to serve.