Preheat oven to 400 degrees. Cover a baking dish or baking sheet with aluminum foil and set aside.
Cut the squash in half. Use a spoon to scrape out the seeds and stringy bits inside each half. Brush 1 tablespoon olive oil over the inside of each squash half and sprinkle with salt and pepper.
Place the squash halves cut-side down on a baking sheet and roast in the preheated oven for 35-40 minutes, or until the squash is tender when pierced with a fork. Set aside to cool slightly.
Sausage Filling
While the squash is roasting, heat the remaining 1 tablespoon of olive oil in a large skillet over medium heat. Add the celery and onion, sautéing for about 5 minutes until softened.
1/2 cup Diced Celery, 1/2 cup Onion
Add the Italian sausage to the skillet, breaking it up with a wooden spoon as it cooks. Brown until sausage is fully cooked through. Stir in the diced Granny Smith apple and cook for an additional 3-4 minutes until the apple softens slightly. Remove the skillet from the heat.
1 cup Diced Apple, 1 pound Italian Sausage
Add sour cream and beaten egg to the sausage mixture and mix until all ingredients are incorporated.
1 Egg, 1/2 cup Sour Cream
Fill each acorn squash with the sausage filling and sprinkle with grated cheese. Return the squash to the oven for 20 minutes.
1/2 cup White Cheddar Cheese
Serve immediately.
Notes
If there is any extra sausage filling, bake in a ramekin alongside the squash.