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Cheesy Root Vegetable Gratin

Cheesy root vegetable gratin with thinly sliced Yukon gold, sweet potato, and parsnip layered tight, then baked until bubbling and golden under a Gruyère-Parmesan crust. Warm cream sauce with garlic and thyme seeps into every layer for a creamy, fork-tender bake.
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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: 620
Ingredients Equipment Method Notes

Ingredients
  

Vegetables
  • 1 lb Yukon gold potatoes
  • 1 lb sweet potatoes
  • 2 parsnips
Cream mixture
  • 1.5 cup heavy cream
  • 0.5 cup whole milk
  • 3 garlic minced
  • 1 tsp fresh thyme leaves
  • 1.5 tsp salt
  • 0.5 tsp black pepper
  • 0.25 tsp nutmeg
Cheese topping
  • 2 cup shredded Gruyere divided
  • 0.5 cup Parmesan grated

Equipment

  • 1 sheet pan

Method
 

Prep and preheat
  1. Preheat the oven to 375F, then butter a 9x13-inch baking dish so the gratin releases cleanly after baking.
Warm the cream sauce
  1. In a saucepan over low heat, warm the heavy cream, whole milk, minced garlic, fresh thyme leaves, salt, black pepper, and nutmeg until steaming, not boiling.
  2. Remove the saucepan from the heat so the warm cream mixture stays silky before layering and pouring.
Layer the vegetables
  1. Layer a third of the sliced Yukon gold potatoes, sweet potatoes, and parsnips in the baking dish, then top with a third of the shredded Gruyere.
  2. Repeat the layers twice to build up the gratin, finishing with a third layer of shredded Gruyere on top.
Bake covered
  1. Pour the warm cream mixture evenly over the layered vegetables so it settles between slices.
  2. Cover with foil and bake for 45 minutes to soften the vegetables and start the creamy bake.
Finish and brown
  1. Uncover the dish, sprinkle with the grated Parmesan, and bake 20-25 minutes until bubbly, golden, and fork-tender.
Rest and serve
  1. Rest for 10 minutes before serving so the cheese crust sets and the layers hold together.

Notes

For the best texture, slice the potatoes and parsnips very thin so they soften evenly; if your slices vary a lot, rotate the dish halfway through the 45-minute bake. Refrigerate leftovers in an airtight container for up to 4 days. Freeze is not recommended due to the cream sauce texture, but you can reheat gently in a 325F oven until warmed through. For a lower-fat option, use half-and-half in place of the heavy cream (the gratin will be slightly less rich).