Ingredients
Equipment
Method
Prep and heat the dish
- Preheat the oven to 375°F and butter a 9x13-inch baking dish so the gratin releases easily. A hot oven and a lightly greased dish help the top brown.
Make the cream mixture
- Warm the heavy cream, whole milk, minced garlic, thyme leaves, salt, black pepper, and nutmeg until steaming, then remove from heat. You want steam coming off the surface, not a rolling boil.
Layer the vegetables
- Layer a third of the Yukon gold potatoes, sweet potatoes, and parsnips in the dish, then sprinkle on a third of the Gruyere cheese. Spread evenly so the layers cook through at the same pace.
- Repeat the layers twice more with the remaining vegetables and Gruyere cheese. Keep the top layer reasonably level for even browning.
- Pour the warm cream mixture evenly over the layered vegetables so it settles between slices. The dish should look sauced but not flooded.
Bake covered, then finish
- Cover with foil and bake for 45 minutes at 375°F. Look for edges starting to soften while the center is still tendering.
- Uncover, sprinkle with Parmesan, and bake for 20 to 25 minutes at 375°F until bubbly, golden, and fork-tender. The top should be browned and gently bubbling all over.
Rest and serve
- Rest the gratin for 10 minutes before serving so the cream thickens slightly. The layers should slice more cleanly after resting.
Notes
Pro tip: slice the potatoes and parsnips as evenly as possible so everything turns fork-tender at the same time. Store covered in the refrigerator for up to 4 days; reheat in a 325°F oven until hot throughout. Freezing is not recommended because the cream may loosen after thawing. For a lighter option, replace half the heavy cream with extra whole milk to reduce richness while keeping the gratin creamy.