Preheat oven to 375 degrees F. Measure out 1 cup of the marinara sauce and set aside.
Trim ends from zucchini and slice in half, lengthwise. Use the tip of a spoon to scrape out the zucchini flesh (see recipe note), chop it and set it aside. Place the scooped out zucchini boats into a 13" x 9" baking dish and add about 1" of water. Cover with foil and bake in preheated oven for 20 minutes, until just slightly fork-tender. Remove from oven, and set aside to cool slightly.
Meanwhile, add olive oil to a 12" skillet and place over MEDIUM heat. Add turkey (or beef), onion, bell pepper, and garlic to the skillet. Cook, stirring to break up the ground meat as it cooks. Continue to cook and stir until meat is thoroughly cooked and veggies have softened. Drain off as much grease as possible and return the skillet to the heat.
Add the chopped zucchini flesh, mushrooms, remaining marinara in jar (reserve 1 cup for later), parsley, Italian seasoning, salt, fresh ground black pepper, and crushed red pepper. Stir well to combine. Bring to a simmer, then reduce heat and allow to simmer over low heat for about 15 minutes.
Remove partially cooked zucchini from baking dish and pour off water. Pour the 1 cup of reserved marinara sauce into the empty baking dish and place zucchini boats on top of the sauce.
Divide the filling between the zucchini boats. If you have extra filling, spoon it into the dish around the zucchini boats. Cover the dish with foil, and bake for 20 minutes, or until zucchini is fork tender. Remove dish from oven and set oven to BROIL.
Remove foil and sprinkle zucchini boats with mozzarella, Parmesan and the panko bread crumbs. Place dish in oven, watching closely, and cook for just a couple of minutes until cheese has melted and bread crumbs are golden brown.
My tip for scraping out zucchini flesh. Use a sharp knife to lightly score the zucchini to outline the area you want to scoop out being very careful not to cut through. You just want an outline. Use the tip of a spoon to scrape the flesh out and on to a cutting board.