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A golden-brown baked butternut squash and sweet potato gratin topped with melted cheese and fresh thyme in a white baking dish.

Butternut Squash and Sweet Potato Gratin

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Prep Time 20 minutes
Cook Time 1 hour
Servings: 8
Course: Side Dish
Cuisine: Mediterranean
Calories: 190
Ingredients Method Notes

Ingredients
  

  • 2 large sweet potatoes peeled and thinly sliced (1/8 inch)
  • 1 medium butternut squash neck only, peeled and thinly sliced (1/8 inch)
  • 2 tbsp extra virgin olive oil
  • 4 cloves garlic minced
  • 1.5 cups milk or light cream / plant-milk
  • 1 tbsp fresh thyme leaves chopped
  • 1 tsp sea salt
  • 1/2 tsp freshly cracked black pepper
  • 1 pinch ground nutmeg
  • 1/2 cup shredded Mozzarella cheese
  • 1/2 cup grated Parmesan cheese

Method
 

  1. Prep: Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish with olive oil.
  2. Slice: Using a mandoline or sharp knife, slice the sweet potato and butternut squash into uniform rounds.
  3. Infuse Sauce: In a small saucepan over medium-low heat, add olive oil and garlic. Sauté for 1 minute until fragrant. Add milk, thyme, salt, pepper, and nutmeg. Simmer for 2 minutes (do not boil). Remove from heat.
  4. Layer: Arrange the vegetable slices in the greased dish, overlapping them in an alternating pattern. Pour the warm milk mixture evenly over the top.
  5. Bake Covered: Cover tightly with foil and bake for 40 minutes until vegetables are fork-tender.
  6. Bake Uncovered: Remove foil, sprinkle mozzarella and parmesan cheeses on top, and bake for another 15-20 minutes until golden brown and bubbling.
  7. Rest & Serve: Let rest for 10-15 minutes before serving to set the sauce.

Notes

  • Mandoline Slicing: Aim for $1/8$-inch ($3text{ mm}$) thickness. If slicing by hand, keep slices uniform so they cook at the same rate.
  • Milk Scorch Warning: Heat the milk mixture on low. Do not let it boil, or it may curdle when baked.
  • The Foil Trick: Don't skip the covered baking phase. It steams the hard root veggies tender before you crisp the cheese.
  • Starch Thickness: If your milk is thin (like skim or almond), whisk 1 tsp of cornstarch or flour into the cold milk before heating to help thicken the pan juices.
  • Make-Ahead: You can assemble the dish 24 hours in advance and refrigerate it. Add $5text{ to }10$ minutes to the covered baking time if baking straight from the fridge.