Roasting the onions and garlic give this vinaigrette a nice mellow flavor.
Preheat oven to 425 degrees.
In a small baking dish, toss onion and garlic with 2 tablespoons olive oil. Season to taste with salt and pepper. Roast onions for 30 to 45 minutes or until lightly charred and tender.
In a food processor mince the onion mixture then remove 2- 4 tablespoons and set that aside. Next, purée the onion mixture. Add balsamic vinegar, maple syrup, basil, and mustard.
With processor running add the remaining 1 cup olive oil in a slow, steady stream until dressing is emulsified. Add the minced onion mixture you set aside and give it a stir with a spoon. Season to taste with salt and pepper.
Transfer to a sealed container and refrigerate until needed. Will keep up to one week.