The Provençal version of this gratin calls for a French pumpkin called potiron, but a combination of acorn and butternut squash will make a good substitute.
- 2 large butternut squash (5–6 lbs.), peeled, seeded, and cut into 1 1/2" chunks
- 1 large acorn squash (about 2 lbs.), peeled, seeded, and cut into 1 1/2" chunks
- 3 medium Yukon Gold potatoes (about 1 3/4 lbs.), peeled, halved lengthwise, and thickly sliced
- 8 cloves garlic, chopped
- 2 cups milk
- 4 cups grated Gruyère
- Kosher salt and freshly ground black pepper, to taste
Preheat oven to 400°. Put the butternut and acorn squash into a large pot, cover with salted water, and bring to a boil. Reduce heat to medium-low and simmer, partially covered, until squash are tender, about 20 minutes. Drain well, pressing out any excess water. Meanwhile, put potatoes into a large pot, cover with salted water, and bring to a boil. Reduce heat to medium-low and simmer until just tender, about 10 minutes. Drain.
Put squash into a large bowl and mash coarsely with a potato masher. Add the drained potatoes, garlic, and salt and pepper to taste and gently fold together. Transfer squash mixture to a deep 9" × 14" roasting pan or baking dish. Pour milk evenly over top and sprinkle with cheese. Bake until golden brown and bubbly, about 30 minutes. Let cool slightly before serving.