Maple Sweet Potato Puree with Caramelized Onions

large sweet potatoes, peeled and chopped into rough chunks

grade B maple syrup, plus 2 tablespoons

extra-virgin olive oil, divided in 1/2

kosher salt, plus more for seasoning

freshly ground black pepper, plus more for seasoning

butter

large Vidalia or other sweet onion, thinly sliced

Hot water