Preheat oven to 350° Fahrenheit. Lightly grease bottom and sides of casserole dish with butter or neutral cooking spray. Set casserole dish aside.
Place large skillet on stovetop over medium-high heat. Add ½ pound ground sweet Italian sausage to skillet and cook, stirring occasionally to break up sausage, until meat is crumbled and just browned, approximately 8 to 10 minutes.
Drain any liquid from skillet then return skillet to heat. Add 1 tablespoon olive oil and 4-6 ounces thinly sliced white onion to skillet with crumbled sausage and stir to incorporate. Sauté mixture, stirring occasionally, until onions are softened, approximately 2 to 3 minutes.
When onions have softened, add 2 tablespoons chopped fresh sage, 3 cloves garlic, 2 cups chopped kale, and ¼ cup low-sodium chicken stock. Stir to incorporate all ingredients, then simmer mixture, stirring occasionally, 3 to 5 minutes or until kale softens and broth reduces slightly.
Place 4 cups cubed butternut squash in casserole dish, then pour sausage-kale mixture over butternut squash. Season with salt and freshly ground black pepper to taste, then gently stir or toss ingredients together until butternut squash is fully incorporated into sausage-kale mixture. Spread mixture out into flat, even layer covering bottom of dish completely, then set casserole dish aside.
Wipe out skillet with paper towels, then return skillet to stovetop over medium-high heat. Add 3 tablespoons unsalted butter and let butter melt completely, swirling and tilting pan occasionally to distribute butter across entire surface.
Once butter is completely melted and just beginning to bubble, add ¾ cup panko breadcrumbs and stir constantly until breadcrumbs are golden brown and lightly toasted.
Remove skillet from heat and let cool slightly, then add ⅓ cup grated fresh parmesan, 1 tablespoon chopped fresh sage, salt, and freshly ground black pepper. Stir gently until parmesan has melted and all ingredients are fully combined.
Transfer breadcrumb mixture to casserole dish and spread evenly over sausage-kale filling, covering filling completely.
Place baking dish in preheated oven. Bake casserole 20 minutes.
After 20 minutes, carefully remove casserole dish from oven. Cover dish with aluminum foil, then return dish to oven and bake 15 to 20 minutes or until squash is tender.
Once squash is tender, carefully remove casserole dish from oven. Uncover casserole dish and discard foil. Let casserole stand 5 minutes, then serve warm.