Halve squash lengthwise and scoop out the seeds. Place facedown in a microwave-safe dish; add water. Microwave, uncovered, on High until the flesh can be easily scraped with a fork, 10 to 15 minutes.
Heat 1 tablespoon oil in a large skillet over medium-high heat. Scrape the squash flesh into the skillet and cook, stirring occasionally, until the moisture is evaporated and the squash is beginning to brown, 5 to 10 minutes. Stir in 1/4 cup parsley. Remove from heat, cover and let stand.
Meanwhile, combine the remaining 1/4 cup parsley, 1/4 cup Parmesan, 1/2 teaspoon Italian seasoning, onion powder, 1/4 teaspoon salt and pepper in a medium bowl. Add turkey; gently mix to combine (do not overmix). Using about 2 tablespoons each, form into 12 meatballs.
Heat the remaining 1 tablespoon oil in a large nonstick skillet over medium-high heat. Add the meatballs, reduce heat to medium and cook, turning occasionally, until browned all over, 4 to 6 minutes. Push the meatballs to the side of the pan, add garlic and cook, stirring, for 1 minute. Add tomatoes, crushed red pepper to taste, the remaining 3/4 teaspoon Italian seasoning and 1/4 teaspoon salt; stir to coat the meatballs. Bring to a simmer, cover and cook, stirring occasionally, until the meatballs are cooked through, 10 to 12 minutes more.
Serve the sauce and meatballs over the squash, sprinkled with the remaining 1/4 cup Parmesan.