Cook pasta according to package directions. Drain and set aside. Keep warm.
Meanwhile, melt butter and olive oil together in a large skillet over medium heat. Add fennel, rosemary and sugar and cook 10 minutes, until fennel is tender and golden brown. Stir in the sweet potatoes.
Whisk together chicken broth, milk, and flour. Gradually add to skillet and simmer 3 minutes, until mixture thickens, stirring constantly. Add the pasta and stir to coat. Stir in Parmesan, parsley, salt, and pepper and cook until heated through, stirring constantly, about 3 minutes.