Preheat the oven to 375 degrees F. Generously grease a shallow 9x13-inch casserole dish or similar 3-quart baking dish with non-stick cooking spray.
Prepare the filling in two batches: Heat 1 tablespoon of the olive oil in a large, deep skillet or Dutch oven over medium heat. Add half of the butternut squash, ½ tablespoon of the rosemary, ¼ teaspoon salt, and 1/8 teaspoon pepper. Sauté until the squash is beginning to brown, about 3 minutes. The squash will still be too firm to eat.
Next, add half of the kale, half of the onions, and half of 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.
Transfer the mixture to the prepared baking dish and with a spoon, prod it into a relatively even layer.
Heat the remaining tablespoon olive oil in the skillet, then repeat the above steps 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 to 35 minutes or until the squash is soft when pierced with a sharp knife.
While the squash bakes, prepare the topping: In a small bowl, combine the breadcrumbs, Parmesan, sage, nutmeg, and salt. Pour the melted butter over the top. Stir to combine.
Remove the casserole from the oven, remove 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.
*You also can omit the kale and make this recipe with a few extra cups of additional butternut squash, though we love the mix of both!
**To make your own fresh breadcrumbs, tear bread (leftover sourdough or similar artisan breads or leftover baguette is my favorite; you can also use 3 slices of sandwich bread) into pieces and place in the food processor. Pulse in long bursts until you have crumbs. Measure and use in the recipe as directed.