Cheesy Winter Squash Gratin

Name: Cheesy Winter Squash Gratin
Ingredients: 5 Tbsp. unsalted butter, divided, plus more for pan; 3½ lb. delicata and/or acorn squash, halved, seeds removed, cut crosswise ¼ thick; 1 large white onion, thinly sliced; 10 garlic cloves, thinly sliced; 2¼ tsp. kosher salt, divided; 6 oz. frozen kale, spinach or blanched greens, thawed, squeezed well; ½ tsp. crushed red pepper flakes; 1 Tbsp. thyme leaves, plus sprigs; 2 cups heavy cream; 4 oz. Parmesan, finely grated, divided; 6 oz. sliced rye or country-style bread, torn into ½ pieces (about 3 cups)