Prepare the Zucchinis: Wash the zucchinis thoroughly and cut off the stems. Slice each zucchini in half lengthwise, creating boat-shaped halves. Gently scoop out the seeds and interior of the zucchinis with a spoon, leaving a zucchini boat.
Cook the Filling: In a skillet, heat oil over medium heat. Add diced onions and minced garlic, sautéing until translucent. Add ground chicken to the skillet, breaking it apart and cooking until browned.
Stir in taco seasoning, salt, and pepper to taste. Add marinara sauce, combining well, and cook for an additional 5 minutes.
Optional Bechamel Sauce: In a separate saucepan, whisk together milk, cornstarch, garlic powder, salt, and pepper. Heat the mixture over medium heat, stirring continuously until it thickens. Set aside.
Assemble and Bake: Preheat the oven to 375°F (190°C). Place the zucchini boats in a baking dish. Fill each boat generously with the ground chicken mixture. Optional: Pour the prepared bechamel sauce over the zucchini boats. Bake in the preheated oven for 20-25 minutes or until the zucchinis are tender.
Serve: Remove from the oven and let it cool for a few minutes. Garnish with fresh herbs or green onions if desired. Serve the zucchini boats with a side salad or your favorite condiments.