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Cheesy Root Vegetable Gratin

Cheesy root vegetable gratin with thin-sliced Yukon gold potatoes, sweet potatoes, and parsnips, layered for even tenderness. Baked at 375°F in a creamy garlic-thyme milk mixture, then finished with a bubbling Gruyère and Parmesan crust.
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Prep Time 25 minutes
Cook Time 1 hour 10 minutes
resting 10 minutes
Total Time 1 hour 45 minutes
Servings: 8 servings
Course: Side Dish
Cuisine: French-inspired
Calories: 690
Ingredients Equipment Method Notes

Ingredients
  

Yukon gold potatoes
  • 1 lb Yukon gold potatoes peeled and thinly sliced
sweet potatoes
  • 1 lb sweet potatoes peeled and thinly sliced
parsnips
  • 2 parsnips peeled and thinly sliced
heavy cream
  • 1.5 cup heavy cream
whole milk
  • 0.5 cup whole milk
garlic
  • 3 clove garlic minced
fresh thyme leaves
  • 1 tsp fresh thyme leaves leaves only
salt
  • 1.5 tsp salt
black pepper
  • 0.5 tsp black pepper
nutmeg
  • 0.25 tsp nutmeg
shredded Gruyère
  • 2 cup shredded Gruyère divided
Parmesan
  • 0.5 cup Parmesan grated

Equipment

  • 1 sheet pan

Method
 

Prep and heat
  1. Preheat the oven to 375°F, then butter a 9x13-inch baking dish.
  2. Warm the heavy cream, whole milk, minced garlic, fresh thyme leaves, salt, black pepper, and nutmeg over medium heat until steaming, then remove from the heat.
Layer and bake
  1. Layer a third of the Yukon gold potatoes, sweet potatoes, and parsnips in the baking dish, then top with a third of the shredded Gruyère.
  2. Repeat the layers twice, then pour the warm cream mixture evenly over the top.
  3. Cover with foil and bake at 375°F for 45 minutes.
  4. Uncover, sprinkle with Parmesan, and bake at 375°F for 20-25 minutes until bubbly, golden, and fork-tender.
Finish
  1. Rest the gratin for 10 minutes before serving to help it set and slice cleanly.

Notes

For the cleanest bake, slice all vegetables to a consistent thin thickness so they finish at the same time. Refrigerate leftovers covered for up to 4 days; reheat in a 325°F oven until warmed through. Freezing is possible, but the creamy texture may soften when thawed and reheated. For a lighter option, use half-and-half instead of the heavy cream for a slightly less rich gratin.