Combine sliced shallot, red wine vinegar, and granulated sugar in a small bowl.
Let stand 1 hour.
Finely chop remaining shallot.
Combine with pork and next 11 ingredients (through egg white) in a large bowl. With wet hands, divide mixture into 40 portions (1 scant tablespoon each).
Roll each into a 1-inch ball. Arrange on a jelly-roll pan lined with parchment paper.
Bake at 375 for 16 minutes.
Preheat broiler to high. Broil meatballs on middle rack 4 minutes or until tops begin to brown.
While meatballs cook, bring a saucepan of water to a boil. Using a paring knife and starting from the root end, carefully remove outer leaves from the Brussels sprouts, setting aside 40 large leaves. Reserve cores for another use.
Add leaves to boiling water; cook 1 minute.
Drain. Immediately place leaves in a bowl of ice water.
Drain. Pat leaves dry with paper towels.
Arrange 1 meatball in each Brussels sprout "cup." Top each with 1 shallot slice.
