Preheat the oven to 400 degrees F (200 degrees C). Spray a rimmed baking sheet with nonstick spray.
Drizzle the inside of spaghetti squash with olive oil and sprinkle with salt and black pepper. Place squash flesh-side down on the prepared baking sheet.
Bake in the preheated oven until fork-tender, about 1 hour. Let cool until safe to handle. Use a fork to scrape strands across the width of the squash into a bowl.
Heat a large skillet over medium-high heat. Cook and stir ground beef, garlic, and dried onion in the hot skillet until browned and crumbly, 5 to 7 minutes. Add pizza sauce to the skillet; mix to combine and set meat sauce aside.
Combine cream cheese, sour cream, and parsley in a small mixing bowl and stir well.
Preheat the oven to 350 degrees F (175 degrees C). Coat a 9x13-inch baking dish with nonstick spray.
Place 1/2 the cooked spaghetti squash into the prepared baking dish. Top with 1/2 the meat sauce and all of the cream cheese mixture. Add remaining spaghetti squash followed by remaining meat sauce. Sprinkle shredded Italian cheese blend on top.
Bake in the preheated oven until cheese is melted and casserole is hot, about 20 minutes.
