Sweet Potato and Kale Soup with Tiny Meatballs

1 Tbsp. extra-virgin olive oil 1/2 cup finely chopped onion 2 large sweet potatoes, or several small ones, peeled and diced 2 garlic cloves, minced 1 tsp. chopped fresh rosemary leaves 1 1/2 lb. kale, washed, dried, and chopped into Read on!