Slice the zucchinis in half lengthwise and scoop out some of the flesh to create boats. Be careful not to go all the way through.
In a skillet, heat the olive oil over medium heat. Add the minced garlic and sauté until fragrant, about 1 minute.
Stir in the shredded chicken, marinara sauce, Italian seasoning, salt, and pepper. Cook until heated through, about 3–5 minutes.
Take each zucchini half and spoon the chicken mixture into the hollowed-out center, packing it tightly.
Top each filled zucchini with shredded mozzarella and grated Parmesan cheese.
Place the zucchini boats on a baking sheet and bake in the preheated oven for 20–25 minutes, or until the zucchinis are tender and the cheese is melted and bubbly.
If desired, broil for an additional 2-3 minutes for a golden cheese topping.
Remove from the oven and let cool slightly before serving. Garnish with fresh basil leaves if using.