Total time: 20 mins
Gives 48 rounds
1 loaf thin sandwich bread
1 cup mayonnaise
1/2 cup freshly grated Parmigiano Reggiano DOP
Yellow mustard, for spreading
10 to 12 small white onions (pearl or boiling onions), very thinly sliced
Preheat the broiler to 220 degrees C.
With a 6-7 cm diameter cookie cutter, cut 2 rounds out of each slice of bread. Arrange the rounds on a baking sheet and place 15 cm from the heat source. Toast the bread on one side until they are golden brown, 3 to 5 minutes.
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.
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.