lightly adapted from Bon Appétit.
- 5 Tablespoons unsalted butter, divided, plus more for pan
- 3½ pounds delicata and/or acorn squash, halved, seeds removed, cut crosswise ¼” thick
- 1 large white onion, thinly sliced
- 10 garlic cloves, thinly sliced
- 2 teaspoons kosher salt, divided
- 12 ounces frozen kale, spinach or blanched greens, thawed, squeezed well
- ½ teaspoon crushed red pepper flakes
- 1 Tablespoon thyme leaves, plus sprigs
- 2 cups heavy cream
- 4 ounces Parmesan, finely grated, divided
- 6 ounces sliced rye or country-style bread, torn into ½” pieces (about 3 cups)
- Place racks in upper and lower thirds of oven; preheat to 400°F. Butter a 13×9″ baking dish. Heat 2 Tbsp. butter in a large high-sided skillet over medium. Add onion, garlic, and ½ tsp. salt and cook, stirring often, until onion is softened and starting to turn golden brown, 8–10 minutes. Stir in kale, red pepper flakes, and 1 Tbsp. thyme and cook, stirring, 1 minute. Add the squash pieces to a large bowl and add the onion mixture.
- Combine cream, three quarters of Parmesan, and 1 tsp. kosher salt in skillet (no need to wipe out) and cook, stirring, until hot but not simmering, about 3 minutes. Carefully pour over squash mixture and mix everything together to coat evenly. Pour into the baking dish, pressing everything in so that it fits semi-neatly. Cover tightly with foil, transfer to lower rack of oven, and bake until squash is just shy of tender, 25–30 minutes. Remove foil and continue to cook until cream is thickened and no longer liquid-y and squash is very tender, 18–22 minutes.
- Meanwhile, wipe out skillet and melt remaining 3 Tbsp. butter in skillet over medium heat. Add bread and remaining ¼ tsp. salt; toss to coat. Transfer to top rack in oven and bake, stirring once or twice, until golden brown and crisp, 10–15 minutes. Let cool.
- Heat broiler. Scatter breadcrumbs over gratin, crushing into smaller pieces as you go, then top with remaining Parmesan. Transfer gratin to top rack and broil until bubbling and browned at the edges, about 1-3 minutes. Top with a few thyme sprigs to serve.