Place a rack in lower third of oven and preheat to 500°F.
Arrange sliced onions into 9x12" roasting pan that will hold all onion slices. Blend marinade ingredients in small bowl and pour over onion slices (See Note 1).
Drizzle the onions with olive oil, season with salt and pepper. Roast for 30 minutes or until wedges are well charred and liquid has become syrupy.
Notes
Cover with plastic wrap and refrigerate overnight for a deeper flavor, optional.