Ingredients

• 1 loaf thin sandwich bread
• 1 cup mayonnaise
• 1/2 cup freshly grated Parmesan
• Yellow mustard, for spreading
• 10 to 12 small white onions (pearl or boiling onions), very thinly sliced

Instructions

1. Watch how to make this recipe.
2. Preheat the broiler to 425 degrees F.
3. With a 2 1/8-inch cookie cutter, cut 2 rounds out of each slice of bread. Arrange the rounds on a baking sheet and place 5 inches from the heat source. Toast the bread on one side until they are golden brown, 3 to 5 minutes.
4. Using a whisk, mix together the mayonnaise and Parmesan in a small bowl. Remove the bread from the oven and turn the rounds over to the untoasted sides. Spread each with a thin layer of mustard. Put 1 onion slice on top and cover with about 1 teaspoon of the mayonnaise mixture, spreading the mixture across the onion and bread.
5. Broil until golden, about 1 minute. Never take your eyes off these; the time varies with different ovens. Work in batches of 24 rounds. Serve one batch hot while the next is cooking.