Kale, White Bean, and Sweet Potato Soup

dried cannellini beans

sprigs fresh rosemary

small onion, cut into quarters

head garlic

Salt

tubetti (small tube-shaped pasta)

olive oil

small leeks, rinsed, white and light-green parts cut in half lengthwise and thinly sliced

sweet potato, peeled and cut into 1/2-inch cubes

bunch kale (3/4 pound)

Homemade Chicken Stock , or canned low-sodium chicken broth, skimmed of fat

freshly ground black pepper