Using a glass (I found that a shot glass works great) or a 2-inch round cookie cutter, cut out 2 to 3 bread rounds from each bread slice; discard trimmings or save to Make Homemade Bread Crumbs.
Spread one side of each bread circle with a heavy layer of butter. Place a thin layer of sliced onions over the butter. Lightly season the onions with salt.
Assemble the sandwiches with remaining bread slices, pressing lightly to make them adhere.
Spread some mayonnaise around the outside rim of each sandwich round. Roll the rim of each sandwich in the minced chives or parsley to coat.
Serve immediately or cover and refrigerator for a few hours.
Makes 16 to 24 sandwich rounds.
Making Sandwiches Ahead of Time:
If you need to make tea sandwiches in advance and need to keep them from drying out, cover them loosely with a sheet of wax paper and then place a damp kitchen towel over the wax paper (never place a damp towel directly on top of the bread because the sandwiches will become soggy). Refrigerate.
When ready to serve, remove from refrigerator. Uncover sandwiches just before serving.
* Choose from Vidalia, Walla Walla, Maui, Imperial, Carzalia, or Texas Spring sweet onions. You probably will not use all the onion
James Beard's Onion Sandwich Recipe: https://whatscookingamerica.net/appetizers/onionsandwich.htm