Step 1

Preheat oven to 425ºF.Heat 2 Tbsp. olive oil in a large sauté pan over medium heat. Once hot, add onion; cook for 7 to 8 minutes, until soft. Stir in garlic and red pepper flakes; cook 2 more minutes.

Step 2

Stir in kale and cook until softened, about 2 to 3 minutes. Stir in heavy cream, 1/3 cup Parmesan cheese, Dijon mustard, fresh thyme, salt and pepper; simmer mixture until thickened, about 3 to 5 minutes.

Step 3

Arrange butternut squash in a greased 2- or 3-quart (11" x 7") baking dish. Carefully pour kale mixture over butternut squash, stirring to make sure it gets into all the crevices. Scatter Gruyere cheese overtop.Loosely cover casserole with foil and bake at 425ºF for 20 minutes.

Step 4

Meanwhile, combine sourdough, walnuts, remaining 2 Tbsp. Parmesan, 2 Tbsp. olive oil, and a small pinch of salt in a medium bowl. Massage oil into bread mixture until well-coated.Remove casserole from oven and discard foil. Scatter sourdough mixture overtop and return casserole to the oven. Bake, uncovered, for 15 to 20 more minutes, until the squash is tender and filling is thickened. Broil for the final 1 to 2 minutes for a more golden-brown crust, if desired. Let stand 5 to 10 minutes before serving.