Cheesy Winter Squash Gratin Recipe

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)