Cook squash on a baking sheet in the preheated oven for 40 minutes, or until tender. Shred with a fork once cooled slightly, then transfer to a lightly oiled casserole dish, discarding shell. Do not turn off the oven.
Heat 1 tablespoon of oil in a skillet over medium heat. Cook and stir the onions, mushrooms, and basil until onions are translucent and tender. Stir onion mixture and sour cream into the squash until well mixed. Sprinkle with Parmesan cheese and cover with bread cubes.
Bake in the preheated oven for 15 minutes, or until warmed through and top is lightly browned and toasted. Enjoy!