Ingredients

1. 2 medium butternut squash (about 3 pounds)--peeled, halved, seeded and sliced crosswise 1/16 -inch thick
2. Salt and freshly ground pepper
3. 4 medium Idaho potatoes (about 1 1/2 pounds), peeled and sliced 1/16 inch thick
4. 5 medium tomatoes, peeled and thinly sliced crosswise
5. 3 1/2 cups heavy cream
6. 1 tablespoon finely chopped garlic
7. 1 teaspoon finely chopped thyme
8. 1/2 teaspoon freshly grated nutmeg
9. 1 1/2 cups fresh bread crumbs
10. 1/4 cup freshly grated Parmesan cheese
11. 2 tablespoons finely chopped parsley
12. 1 tablespoon finely chopped chives

Directions

1. Preheat the oven to 350°. Butter a 14-by-10-by-2-inch (4 quart) baking dish. Layer half of the butternut squash slices in the baking dish, overlapping them slightly, and season with salt and pepper. Cover with half of the potato slices, overlapping them slightly, and season with salt and pepper. Top with half of the tomato slices and season with salt and pepper. Repeat the layering process with the remaining vegetables, seasoning as you go.
2. In a medium saucepan, combine the cream, garlic, thyme and nutmeg and bring to a boil over moderate heat. Pour the cream evenly over the vegetables. Cover with foil and bake for about 45 minutes, or until the vegetables are tender when pierced with a knife. 
3. Meanwhile, in a small bowl, combine the bread crumbs, Parmesan, parsley and chives. Sprinkle the crumbs evenly over the gratin. Increase the oven temperature to 400° and bake the gratin, uncovered, for 20 to 25 minutes, or until the crust is golden brown. Let rest for 10 minutes before serving. 

Make Ahead

The gratin can be refrigerated overnight. Rewarm, covered, in a 300° oven for 45 minutes. Uncover and bake until the top is crisp.