Sweet Potato and Kale Grilled Cheese

medium sweet potato, peeled and sliced into thin rounds

olive oil, divided

Salt and black pepper,

small red onion, sliced

butter

slices whole grain bread

Harvarti cheese, sliced

leaves Lacinato (Tuscan or Dino) kale, torn into pieces

fresh rosemary, minced

Salt and black pepper,

Balsamic glaze or reduction, if desired