Step 1

Preheat oven to 375 degrees F (190 degrees C). Grease a baking sheet or line with aluminum foil.

Step 2

Place squash, cut sides down, on the prepared baking sheet.

Step 3

Bake in the preheated oven until tender, about 1 hour. Flip squash halves over and set aside to cool.

Step 4

Heat a large skillet over medium-high heat. Cook and stir sausage in the hot skillet until browned and crumbly, 5 to 7 minutes; drain and discard grease. Add celery, mushrooms, and onion; cook and stir until celery is softened, 5 to 10 minutes. Remove skillet from heat.

Step 5

Beat egg in a large bowl; stir in sour cream and Parmesan cheese. Stir cheese mixture into sausage mixture. Fill squash halves with sausage filling.

Step 6

Bake in the preheated oven until filling is cooked through, about 20 minutes.