Preheat your oven to 400°F. While the oven heats, prepare the zucchini by cutting each one in half lengthwise. Using a small spoon or melon baller, carefully scoop out the centers to create ¼-inch shells, leaving enough structure so the boats hold their shape during baking. Arrange the hollowed zucchini halves cut-side up on a baking sheet or rimmed baking tray.
In a large mixing bowl, combine the shredded chicken, ricotta, alfredo sauce, and 4 tablespoons of the parmesan cheese. Stir until everything is evenly incorporated and creamy. Add the minced garlic, 1½ teaspoons Italian seasoning, salt, pepper, red pepper flakes, and lemon zest. Mix thoroughly to distribute the seasonings throughout the filling—I find that stirring in one direction helps the ingredients blend more smoothly without overworking the mixture.
Gently fold the roughly chopped spinach into the filling mixture just until combined—I like to stop folding while some spinach ribbons are still slightly visible, as this helps preserve texture and prevents the filling from becoming too dense. Spoon the filling evenly into each hollowed zucchini boat, using a spatula or the back of a spoon to pack it gently into place and smooth the tops.
Place the filled zucchini boats in the preheated 400°F oven and bake for 15 minutes. This allows the zucchini to soften, the filling to heat through, and the flavors to meld together. The zucchini should still have some firmness and shouldn't collapse.
Remove the zucchini boats from the oven and evenly distribute the shredded mozzarella over the top of each boat. Sprinkle the remaining 1 tablespoon of parmesan cheese and ½ teaspoon Italian seasoning over the mozzarella. Return to the oven and bake for 3–5 minutes until the cheese melts and lightly browns in spots—watch carefully during this step as the cheese can brown quickly.
Remove the zucchini boats from the oven and let them rest for 2–3 minutes before serving. This brief resting period allows the cheese to set slightly and the filling to stabilize, making the boats easier to plate and eat. Serve warm.