Gather all ingredients. Preheat the oven to 375 degrees F (190 degrees C).

Brush the inside of squash halves with olive oil; season with salt and pepper. Place cut-sides up on a baking sheet.

Bake in the preheated oven until softened, about 45 minutes. Remove from the oven and let cool until easily handled, 5 to 10 minutes.

Scrape out strands of squash with a fork and transfer them to a bowl. Adjust the oven temperature to 350 degrees F (175 degrees C).

Mix 1/2 of the mozzarella cheese, ricotta cheese, egg, and Parmesan cheese together in a bowl. Stir into squash. Stir in 2 or 3 tablespoons spaghetti sauce to turn mixture pink.

Spread 1/2 of the squash mixture in a 9x13-inch baking dish. Spoon 1/2 of the spaghetti sauce on top. Sprinkle with 1/2 of the remaining mozzarella cheese. Layer remaining squash mixture, spaghetti sauce, and mozzarella cheese on top.

Bake in the preheated oven until mozzarella cheese is melted and browned, 30 to 45 minutes.