First mix the vinegar, onion powder, sugar, salt and pepper. Mince the garlic clove, or use a garlic press and add the garlic to the marinade. Mix the marinade well. (As a short cut, you can use Good Seasons Italian dressing mix. Follow the recipe on the package. Use balsamic vinegar but leave out the oil.)
Wash the asparagus in cool running water.
Cut the ends off until only the green tender parts remain, usually about an inch or so off of the bottom. You can also snap the ends off one by one, as the asparagus will usually snap right where the tender part starts, but this is much more time consuming.
Place the asparagus in a bowl and pour the marinade over. Cover and refrigerate for one hour.
Heat the grill, medium heat.
Remove asparagus from the marinade and drizzle the olive oil over the stalks. Place the asparagus on the grill so they are perpendicular to the grates. Spread them out so they all are exposed to the heat, but are snug up against each other. Grill for 2-3 minutes over the direct flame with the lid open until they begin to brown slightly (ever so slightly).
Turn the asparagus careful. It is best to use tongs. Grab only a few at a time or you will risk loosing some stalks down between the grates.
Grill for another 2-3 minutes until a nice grill color appears.
Pull one off and taste to ensure they are cooked to your level of desired tenderness. Leave on for a few minutes more if you like, but don’t overcook.
Serve as is, or add a little butter or hollandaise sauce.