Preheat the oven to 375°. Spray a baking sheet with cooking spray and set aside.
FOR THE ZUCCHINI:
Slice the zucchini lengthwise down the middle. Use a melon-baller to carefully scoop the flesh out of the zucchini and transfer to a mini prep food chopper. Pulse several times until the zucchini flesh is finely chopped. Squeeze out any excess moisture from the zucchini. Set aside.
Heat a large skillet over medium heat and add 1 tablespoon of the olive oil. Add the chopped onion and garlic and sweat the vegetables for 3-4 minutes until they're tender and translucent.
Add the chopped zucchini, crushed red pepper, oregano, salt and pepper and cook for an additional 3-4 minutes until very fragrant. Transfer the vegetables to a separate dish.
Remove the chicken sausages from their casings.
Heat the remaining olive oil in the skillet over medium high heat and add the sausage, breaking it up with a spoon and cooking until it's nearly cooked through. Add the zucchini and onion mixture and cook, stirring constantly until the meat is browned and excess moisture has evaporated.
Stir in half of the breadcrumbs and fold them into the sausage mixture.
Arrange the zucchini halves onto a rimmed baking sheet and divide the sausage filling between them. Top with the grated mozzarella, parmesan and the panko breadcrumbs.
Bake for 20-25 minutes or until the panko is golden and crispy and the cheese has melted.
Sprinkle with fresh parsley and serve with marinara sauce if desired.