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Cheesy Root Vegetable Gratin

Cheesy root vegetable gratin with thin-sliced Yukon gold potatoes, sweet potatoes, and parsnips baked until fork-tender under a bubbling golden Gruyère crust. Warm cream-milk-garlic thyme sauce soaks into every layer for a creamy, sliceable gratin.
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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: 520
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
salt
  • 1.5 tsp salt
black pepper
  • 0.5 tsp black pepper
nutmeg
  • 0.25 tsp nutmeg
shredded Gruyere
  • 2 cup shredded Gruyere divided
Parmesan
  • 0.5 cup grated Parmesan

Equipment

  • 1 sheet pan

Method
 

Prep and preheat
  1. Preheat the oven to 375F, then butter a 9x13-inch baking dish. Make sure the dish is well coated so the gratin releases cleanly after baking.
  2. Warm the heavy cream, whole milk, minced garlic, fresh thyme leaves, salt, black pepper, and nutmeg until steaming, then remove from the heat. Look for small bubbles and steam rising from the surface.
Layer and bake
  1. Layer one third of the Yukon gold potatoes, sweet potatoes, and parsnips in the baking dish, then top with one third of the shredded Gruyere. Spread the cheese evenly so it melts into the vegetables.
  2. Repeat the layers two more times with the remaining vegetables and shredded Gruyere. Aim for even thickness so everything bakes at the same rate.
  3. Pour the warm cream mixture evenly over the top of the layered vegetables. The surface should look glossy and slightly submerged.
  4. Cover the dish with foil and bake for 45 minutes at 375F. You should see bubbling around the edges once it’s actively heating through.
  5. Uncover, sprinkle with the grated Parmesan, and bake for 20-25 minutes at 375F until bubbly, golden, and fork-tender. The top should be deeply browned and the vegetables should offer no resistance when pierced.
Rest and serve
  1. Rest the gratin for 10 minutes before serving. It will thicken as it cools, making clean slices and a creamy texture.

Notes

Pro tip: slice the potatoes, sweet potatoes, and parsnips very thin and uniform (about the same thickness) so the bake time stays consistent. Store leftovers in the refrigerator up to 4 days; reheat in a 350F oven until warmed through. Freezing is not recommended because the cream sauce can break and the texture may turn grainy. For a lighter option, replace whole milk with lactose-free milk or use part-skim dairy if needed, and keep the same bake times.