Penne with Lemon and Root Vegetables

Name: Penne with Lemon and Root Vegetables
Ingredients: 1 tablespoon extra-virgin olive oil; 5 cups 2 x 1/2 x 1/2-inch sticks peeled assorted root vegetables (such as parsnips, carrots, celery root, and golden beets); 2 large garlic cloves, peeled; 2 celery stalks, cut crosswise into 1/2-inch-wide pieces, plus 6 tablespoons chopped celery leaves, divided; 8 ounces penne rigate or whole grain penne; 3/4 cup finely grated Parmesan cheese; 1 tablespoon finely grated lemon peel; 1/4 teaspoon (scant) ground nutmeg