Roasted Sweet Potato Pear Soup

Name: Roasted Sweet-Potato-Pear Soup
Ingredients: 2 large sweet potatoes; 3 large, firm pears (any type); 1 medium onion; 1 tablespoon olive oil; 2 tablespoons butter, cut into small pieces; 1 tablespoon brown sugar; 1 clove garlic, minced; 4-6 cups chicken or vegetable stock; 1 cup diced cooked ham, optional