Preheat oven to 350°F. Scrub sweet potato and prick all over with a fork. Place potato in a microwave-safe dish. Cover with a microwave-safe lid or with vented plastic wrap. Microwave on 100% power (high) about 7 minutes or until potato is tender. Set potato aside until cool enough to handle.
Meanwhile, in a large pot of salted water, cook the macaroni according to package directions for al dente. Drain and set aside.
When sweet potato is cool enough to handle, peel potato and mash until smooth. Cover and keep warm.
Melt the butter in a large pot over medium-high heat. Whisk in the flour and continue to cook, whisking constantly, until the roux is just beginning to color, about 1 minute. Slowly add the half and half, whisking constantly, until the mixture begins to simmer and thicken.
Remove from heat and add the cheddar cheese by the handful, stirring the sauce until the cheese has melted before adding the next handful. Add the cream cheese cubes and stir until completely melted. Stir in the mashed sweet potato, sage, and ground mustard until combined. Season with 3/4 tsp.salt and 1/4 tsp. pepper, then stir in the macaroni.
Butter a 3-quart broiler-proof baking dish. Transfer pasta to prepared dish and sprinkle the top generously with grated Parmesan cheese. Bake for 10 to 12 minutes, until just beginning to set, then broil the top 1 to 2 minutes to brown. Sprinkle with a few sage leaves before serving, if desired.
Serve this as a main dish for vegetarians, or as a special side for those avoiding gluten.