Serves 4 to 6 INGREDIENTS 1 tablespoon extra-virgin olive oil 1 tablespoon unsalted butter 1 pound onions, roughly chopped (about 4 cups) 4 to 6 peeled garlic cloves 2 tablespoons finely chopped fresh ginger Sea salt or kosher salt 2 pounds assorted winter root vegetables, such as rutabaga, carrot, parsnip, turnip, celery root, and sunchokes,…