Preheat oven to 350°.
In a large bowl, whisk together the first 8 ingredients.
Add potatoes and onion; toss to combine.
Transfer mixture to a greased 15x10x1-in. baking pan.
Bake until potatoes start to brown and are tender, 35-40 minutes, turning once.
Transfer to a serving bowl.
Top with basil and, if desired, additional balsamic vinegar.
