Preheat oven to 400 degrees. Scoop out the insides of each zucchini half, leaving a 1/4-inch thick shell creating a "boat". Finely chop up the insides and set aside (you'll use about 1 cup for the filling).
Sprinkle the insides of the boats with kosher salt (about 1 tsp total) and then place upside down on a paper towel or baking sheet to drain excess moisture for 10-15 minutes.
Make the filling: Heat olive oil in a large skillet over medium heat. Add the chicken sausage and cook, breaking it up with a spoon, until browned and no longer pink, 5-7 minutes. Add diced onion, garlic and bell pepper; saute for 3-4 minutes or until softened.
Stir in the chopped zucchini, spinach, tomatoes, italian seasoning, salt, pepper and red pepper flakes (if using). Cook for another 3-4 minutes or until the spinach is wilted and the mixture has thickened slightly. Remove from the heat and stir in the ricotta cheese.
Assemble the boats: Spread about 1/3 cup marinara sauce in the bottom of a 9x13-inch pan. This creates a flavorful base and helps prevent the zucchini from sticking. Place zucchini boats cut side up in the dish. Spoon the filling evenly into each boat, mounding it slightly. Drizzle the remaining 2/3 cup marinara sauce over the top.
In a small bowl, combine the mozzarella cheese, parmesan cheese, and panko breadcrumbs. Sprinkle the mixture generously over the filled boats.
Bake: Cover the dish loosely with foil and bake for 20 minutes. Remove the foil and bake for an additional 10 minutes or until the zucchini is tender, the cheese is melted and bubbly and the topping is golden. For extra browning, broil on high for 1-2 minutes (watch closely to avoid burning).
Serve: Garnish with fresh parsley or basil. Let rest for 5 minutes before serving.