9. Let it rest
Remove the casserole from the oven and let it sit for at least 10 minutes before serving.
This short resting period allows the creamy sauce to thicken slightly and makes the casserole easier to portion.
Serve the potatoes warm as a comforting side dish or make them part of a hearty winter meal.
Variations and Helpful Tips
Make the onions kid-friendly: If large pieces of onion aren't popular at your table, chop the caramelized onions very finely before adding them to the cream mixture. They'll blend more naturally into the sauce while still adding plenty of flavor.
Change the herbs: Thyme can be replaced with fresh parsley, or you can leave the fresh herbs out altogether. A small amount of dried Italian seasoning also works well.
Make it a little lighter: Replace 1 cup of the heavy cream with 1 cup of whole milk. The casserole won't be quite as rich, but it will still have a creamy and comforting texture.
Add more cheese: For an extra-cheesy version, stir about 1/2 cup of shredded mozzarella or mild cheddar into the creamy mixture along with the Parmesan.
Prepare ahead: The casserole can be assembled, covered, and refrigerated for up to 24 hours before baking. If you're starting with a cold casserole, allow roughly 10 to 15 additional minutes of covered baking time.
Turn it into a heartier dish: Add a layer of thinly sliced ham or cooked crumbled bacon between the potatoes before pouring over the cream mixture.
Add vegetables: For a more vegetable-forward version, tuck thinly sliced mushrooms or blanched broccoli florets beneath the creamy sauce.
Using dried thyme: Dried thyme is a convenient substitute for fresh. Since dried herbs have a more concentrated flavor, use about half as much.
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