Preheat the oven to 400°F. Peel and finely slice the sweet potatoes into ⅛ inch rounds and set aside.
Heat the butter over medium heat in a saucepan until melted. Add the garlic and sage and cook for 30 seconds. Whisk in the cornstarch until combined.
Add the cream, milk, salt and pepper. Heat until a soft boil occurs, about 10 minutes. Remove from heat and stir in the Gruyère and Parmesan cheese.
In a 9x13 baking dish, arrange the sweet potato rounds in rows and pour the cheese sauce over top.
Cover the baking dish with aluminum foil and bake for 35 minutes. Remove the foil and bake another 10 minutes or until the cheese is golden. Serve warm.
Notes
Select sweet potatoes that are firm and free from blemishes. You can use either orange or white-fleshed sweet potatoes, depending on your preference.
Slice the sweet potatoes uniformly to ensure even cooking. A mandoline slicer can be helpful for achieving consistent thickness.
Cover the dish with foil or a lid during the initial baking to trap steam and help the sweet potatoes cook through. Remove the cover during the last part of baking for a golden top.
Allow the dish to rest for a few minutes after baking. This helps the flavors meld and makes it easier to serve.