With its scalloped, petal-like edges, arrange the salami in a layered circle. Keep in mind that the sandwiches should be easy to pick up and eat in a few bites without it crumbling or falling apart.
Cut bread into 2 3/4-inch squares; discard scraps. Spread each piece of bread with 1 teaspoon butter and 1 teaspoon mustard. Top each with 2 slices salami. Divide onion slices among sandwiches.