Prep the oven and pan: Preheat your oven to 400°F (200°C). Lightly grease a large baking dish with olive oil or line it with parchment.

Hollow the zucchini: Trim the ends and slice each zucchini lengthwise. Use a spoon to scoop out the centers, leaving about 1/4-inch walls. Chop the scooped zucchini flesh and set it aside.

Season the boats: Brush the zucchini halves with a bit of olive oil. Sprinkle with salt and pepper. Arrange them cut-side up in the baking dish.

Sauté aromatics: Warm 2 tablespoons olive oil in a skillet over medium heat. Add onion (if using) and cook 3 minutes until softened. Stir in garlic and the chopped zucchini flesh. Cook 2–3 minutes to reduce moisture.

Build the filling: Add the cooked chicken, Italian seasoning, red pepper flakes, 1/2 teaspoon salt, and a few grinds of pepper. Stir in 3/4 to 1 cup marinara, just enough to coat without making it soupy. Remove from heat and mix in 1/2 cup Parmesan and 1/2 cup mozzarella.

Fill the boats: Spoon the chicken mixture evenly into each zucchini half. Don’t overpack to the point of spilling, but a generous mound is perfect.

Add sauce and cheese: Drizzle a little extra marinara over the tops if you like them saucier. Sprinkle with remaining Parmesan, then top with the remaining mozzarella.

Bake: Cover loosely with foil and bake 15 minutes. Remove foil and bake another 10–12 minutes, until the zucchini is tender when pierced and the cheese is bubbling and golden.

Finish and serve: Let rest 5 minutes. Garnish with chopped basil or parsley. Taste and add a pinch of salt or a grind of pepper if needed. Serve warm.