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Cheesy Root Vegetable Gratin

Cheesy root vegetable gratin with thin, tender layers of potatoes, sweet potatoes, carrots, and parsnips baked in a garlicky thyme cream sauce. Finished with melty Gruyère, sharp white cheddar, and a crisp Parmesan crust with bubbly golden-brown edges.
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Prep Time 20 minutes
Cook Time 1 hour 10 minutes
resting 15 minutes
Total Time 1 hour 45 minutes
Servings: 10 servings
Course: Side Dish
Cuisine: French
Calories: 520
Ingredients Equipment Method Notes

Ingredients
  

For the vegetables
  • 1 lb Yukon Gold potatoes peeled and sliced ⅛-inch thick
  • 1 lb sweet potatoes peeled and sliced ⅛-inch thick
  • 0.75 lb carrots peeled and sliced ⅛-inch thick on a diagonal
  • 0.75 lb parsnips peeled and sliced ⅛-inch thick on a diagonal
For the cream sauce
  • 3 tbsp unsalted butter
  • 1 shallot medium, finely minced
  • 3 garlic cloves minced
  • 3 tbsp all-purpose flour
  • 2 cup whole milk
  • 1 cup heavy cream
  • 1 tsp salt
  • 0.5 tsp black pepper
  • 0.25 tsp ground nutmeg
  • 1 tbsp fresh thyme leaves plus a few sprigs for garnish
For the cheese
  • 2 cup Gruyère 8 oz, shredded
  • 1 cup sharp white cheddar 4 oz, shredded
  • 0.5 cup Parmesan 2 oz, finely grated

Equipment

  • 1 sheet pan
  • 1 Dutch oven

Method
 

Prep and build the gratin
  1. Preheat the oven to 375°F (190°C) and butter a 9x13-inch baking dish.
  2. Toss the Gruyère and sharp white cheddar together in a bowl and set aside.
  3. Mince the shallot and garlic.
  4. Peel and slice the Yukon Gold potatoes, sweet potatoes, carrots, and parsnips into ⅛-inch-thick slices, slicing carrots and parsnips on a diagonal.
  5. Shingle one-third of the mixed vegetable slices in overlapping rows in the dish, alternating the colors.
  6. Pour one-third of the sauce over the vegetables and sprinkle one-third of the Gruyère-cheddar mix.
  7. Repeat the layers twice more with the remaining vegetables, sauce, and cheese, then sprinkle the Parmesan evenly over the top.
Make the thyme cream sauce
  1. Melt the butter in a medium saucepan over medium heat and cook the shallot for 2 minutes, until softened.
  2. Add the garlic and cook for 30 seconds, until fragrant.
  3. Whisk in the flour and cook for 1 minute, whisking constantly, until it smells lightly toasted.
  4. Slowly pour in the whole milk and heavy cream while whisking so no lumps form.
  5. Simmer, whisking often, for 4–5 minutes at a gentle bubble, until the sauce thickens enough to coat the back of a spoon.
  6. Remove from the heat and stir in the salt, black pepper, ground nutmeg, and thyme leaves, until fully combined.
Bake and finish
  1. Cover the dish tightly with foil and bake at 375°F (190°C) for 45 minutes.
  2. Uncover and bake for another 25–30 minutes until the top is deep golden and bubbling, and a knife slides easily through the vegetables.
  3. Let the gratin rest for 15 minutes so the layers set.
  4. Garnish with thyme sprigs and serve warm.

Notes

For the neatest layers and the best set, keep your slices around the same thickness (⅛-inch) so they finish baking at the same time. Store leftovers covered in the refrigerator for up to 4 days; reheat in the oven at 350°F until steaming. Freezing is possible but may soften the vegetables slightly. For a lighter option, use half-and-half in place of the heavy cream (the sauce will be a bit less rich but still creamy).