Preheat the oven to 375 degrees F. Lightly coat a 11- by 7-inch baking dish with cooking spray.

Pierce the sweet potato all over with a fork. Cook in a microwave until the sweet potato is very tender, about 5 minutes per side.

Cut the sweet potato in half lengthwise and, when cool enough to handle, scoop the flesh into a shallow dish. With the back of a fork, mash the sweet potato until well-mashed and almost smooth.

In a large nonstick saucepan, cook the elbow pasta about 2 minutes less than package directions. Drain and rinse with cold water.

In the same saucepan, heat 1 teaspoon olive oil over medium heat. Add the onion and cook until the onion is tender, 4 to 5 minutes. Add the garlic and sage and cook for 1 minute.

Stir in the flour and cook, stirring, for 2 minutes.

While whisking, gradually pour the vegetable broth and skim milk into the flour mixture. Bring the mixture to a simmer and continue to whisk until the mixture thickens, about 5 minutes.

Stir in the mashed sweet potato, grated cheddar cheese, Dijon mustard, nutmeg, salt and pepper. Heat, stirring, until the cheese is melted.

Add the cooked pasta to the cheese sauce and stir until well combined. Transfer to the prepared baking dish.

In a small bowl stir together the panko breadcrumbs and Parmesan cheese. Sprinkle the mixture over the top of the macaroni and cheese.

Bake until the casserole is bubbling, about 20 minutes. Place the casserole under the broiler for 1 minute, or until the breadcrumbs are golden brown. Watch carefully so that the breadcrumbs don't burn. Serve.