Start by preheating the oven to 420F. Line 2 baking tray with parchment paper.
Bring a large pot of water to a boil and ½ tablespoon of salt to the water.
While the water comes to a boil, peel the potatoes and sweet potatoes before cutting them into 1-inch cubes.
Add the cubed potatoes to the boiling water and cook until both types of potatoes are fork tender.
Drain the potatoes, making sure any excess water has been removed. Mash the potatoes with masher or ricer.
Add the salt, pepper, onion powder, nutmeg, cinnamon, grated parmesan, egg, and melted butter to the mashed potatoes. Use a whisk to mix all ingredients until smooth.
Equip a large piping bag with a large star piping tip. Place some of the Duchess Sweet Potatoes mixture into the piping bag, filling the bag half way at a time.
Pipe the filling by holding the piping bag vertically and holding the tip 1 inch over the tray. Apply pressure to the piping bag in order to shape a mound, about 2 inch wide and 1 ½ - 2 inch tall. Pipe 12 mounds per tray for a total of 24-26 Duchess Potatoes.
Spray the Duchess Sweet Potatoes with the Olive Oil spray. Place one tray at a time into the preheated 420F oven. Bake for 20-25 minutes or until lightly golden. Remove from the oven and let them rest for 10 minutes before serving.