Melt 3 tablespoons butter in heavy large skillet over medium heat. Add chopped onion and sauté until slightly softened, about 4 minutes. Add chopped beet leaves and stems and sauté 5 minutes. Add corn and sauté until vegetables are tender, about 4 minutes. (Can be made 1 day ahead. Cover and chill.)
Cook macaroni in large pot of rapidly boiling salted water just until tender, stirring occasionally. Drain well. Return to pot. Add remaining 2 tablespoons butter and stir to coat. Add whipping cream and chopped ham to vegetable mixture in skillet and bring to boil. Pour over macaroni. Add grated cheddar cheese and stir to melt. Season macaroni to taste with salt and pepper. Transfer macaroni to serving bowl. Sprinkle with chopped fresh parsley and serve.
