Prepare turkey sausage: In a medium bowl, combine sausage ingredients and mix well. Cover and refrigerate for 8 to 24 hours.
Preheat oven to 375°F. With a spoon, scrape out squash seeds and membranes. Rinse seeds and transfer to a sheet of paper towel and let dry completely; discard membranes, On a large baking sheet, place squash cut side up and drizzle with 2 tbsp oil. Season with 1/4 tsp sea salt and 1 tsp black pepper. Transfer to oven and roast until edges are golden and a knife inserts easily into flesh, 30 to 35 minutes. Remove from oven and set aside.
Meanwhile, on a small baking sheet, place seeds in a single layer. Transfer to oven and roast, stirring every 5 minutes, until crisp and golden brown, 15 to 20 minutes. Remove from oven and sprinkle with remaining 1/4 tsp sea salt.
In a large skillet, heat remaining 1 tbsp oil on medium-high. Add turkey mixture and cook, breaking it up into chunks with a wooden spoon, until well browned and cooked through, 8 to 10 minutes. Transfer to a plate and cover to keep warm. Wipe out skillet.
In same skillet, melt butter on medium heat. Add cabbage and cook, stirring, until tender and almost translucent, about 4 minutes. Add bell peppers and onion and cook until onion softens and peppers are tender-crisp, 6 to 8 minutes. Add turkey mixture and garlic and cook for 2 to 4 minutes. Stir well to combine and blend flavors. If desired, season with additional sea salt and black pepper.
Arrange 1 oven rack in top position and preheat broiler on low. Spoon turkey-pepper mixture into squash halves and top with ricotta and seeds, dividing evenly. Transfer to top rack of oven and broil until ricotta just melts, 1 to 2 minutes. Garnish with parsley, dividing evenly.