Make-Ahead: Arrange onions in a steamer basket over boiling water; cover and steam 10 minutes or until tender. Place in a lightly greased 8" square baking dish.
Melt butter in a small saucepan over medium-low heat. Add sugar, syrup, and salt; bring to a boil over medium heat. Reduce heat; simmer 5 minutes. Pour over onions.
Cover and bake at 350° for 25 minutes. Remove from oven, and cool. Cover with aluminum foil, and chill overnight.
When Ready to Serve: Remove aluminum foil from baking dish; cover dish tightly with heavy-duty plastic wrap. Fold back a small edge (or corner) of wrap to allow steam to escape.
Microwave at MEDIUM (50% power) 12 minutes or until thoroughly heated, turning dish once. Sprinkle with parsley, if desired.
Note: A quick and easy way to peel pearl onions is to trim the bottom ends of onions and blanch onions in batches for about 45 seconds in rapidly boiling water. (It's important to blanch in batches so that the water remains at a boil.) Drain onions and immediately place in a bowl of ice water to stop the cooking process. Simply squeeze trimmed end of each onion gently, and the skins will slide right off.