Step 1

Using a 1 1/2-inch cutter, cut 24 rounds from the bread and arrange in 12 pairs. Spread each round with mayonnaise.

Step 2

Slice the onions through the equator, evenly and thinly, and put a slice that fits the bread on half the bread rounds. Salt lightly, then cover with a second round of bread.

Step 3

When sandwiches are assembled, spread remaining mayonnaise on a piece of parchment paper, and have the chopped herbs ready in a bowl.

Step 4

Hold a sandwich, and roll the edge in mayonnaise, then in the chopped herbs, covering the edge. Cover and chill for about 1 hour and up to 6 hours before serving.