2tablespoonscold unsalted butter - cut into small cubes
Preheat the oven to 375 degrees F. Generously grease a shallow 3-quart baking dish with softened butter or cooking spray.
Prepare the squash and sausage filling: Heat 1 tablespoon of the olive oil in a large, deep skillet or Dutch oven over medium heat. Brown and crumble the sausage until fully cooked, about 5 minutes. Remove from the pan and set aside. Being careful not to burn yourself, wipe out the pan with a paper towel.
Cook the butternut squash filling in two batches (this will ensure the ingredients brown properly): Heat 1 tablespoon olive oil in the skillet, then add half of each of the following: the squash, rosemary, salt, and pepper. Sauté until the squash is beginning to brown, then add half of: the kale, the onions, and the garlic. Cook until the kale and onions begin to soften, about 4 minutes. Add the 1/2 cup of the chicken stock, stir, and let cook 1 minute so that the stock reduces slightly. Stir in half of the reserved sausage. Transfer the mixture to the prepared baking dish and press into a relatively even layer.
Heat the remaining tablespoon olive oil in the skillet, then repeat the above step with the remaining half of each of the ingredients. Transfer the rest of the filling to the casserole dish. Cover the dish with aluminum foil and bake for 30 minutes or until the squash is soft when pierced with a sharp knife.
While the squash bakes, prepare the topping: Place the slices of bread in the bowl of a food processor and pulse into crumbs (if using ready-made breadcrumbs, simply add them to the food processor without pulsing). Add the cornmeal, Parmesan, sage, sugar, and salt. Plus a few times to combine. Drizzle in the olive oil, add the butter cubes, and pulse until crumbly and the butter is fully incorporated. Add the egg and pulse a few additional times, until the mixture just begins to clump together. (Alternatively, you can make in a mixing bowl, cutting in the butter by hand. In this case, you will need prepared breadcrumbs).
Remove the casserole from the oven, take off the foil, then sprinkle the top evenly with the breadcrumb mixture. Reduce the oven temperature to 350 degrees F, return the dish to the oven, then bake uncovered, until the bread crumbs are golden, about 20 minutes. Serve warm.