01 -
Set your oven to 375°F (190°C) and let it warm up.
02 -
Coat a large 10 × 14-inch ceramic baking dish generously with cooking spray.
03 -
Mix the gouda and asiago cheeses, rosemary, garlic powder, paprika, black pepper, sage, and salt in a mixing bowl. Keep this blend aside.
04 -
Arrange a flat, even layer of sweet potato slices at the bottom of the oiled dish.
05 -
Spread a layer of the thinly sliced onions on top of the potatoes.
06 -
Scatter half of the seasoned cheese mixture evenly over the onion layer.
07 -
Repeat the layers by adding more potatoes, a second round of onions, and finish with the last potato layer on top.
08 -
In another bowl, whisk together the milk, sour cream, heavy cream, butter, and cornstarch until the mixture is smooth and everything blends together.
09 -
Pour the creamy mixture carefully over the layered potatoes to soak everything evenly.
10 -
Spread the remaining half of the cheese blend across the top layer of potatoes.
11 -
Place the dish in the oven (middle rack) and cook for 1 hour 15 minutes to 1 hour 30 minutes, until the cheese gets golden brown and bubbly, and the potatoes soften easily.
12 -
Let the dish cool down and settle for about 15 minutes before serving.