Wash and cut both ends of each zucchini, then slice them in half lengthwise. Using a small spoon, gently scoop out most of the seedy flesh.
Once the flesh is removed, sprinkle a little salt over zucchini, and place them flesh side down on paper towels to remove excess moisture.
Preheat oven to 375° Fahrenheit.
Heat up olive oil in skillet over medium-high heat. Once hot, swirl skillet around to get it all coated with oil. Add chopped onion and sliced mushrooms. Cook for about 5 minutes or until tender, stirring often with wooden spoon.
Once mushrooms and onion are tender, push them to one side of the skillet, add a little more olive oil if needed, then add ground chicken. Brown the chicken for 2 minutes alone, stirring often, then mix it with mushrooms and onion.
Add spices: onion and garlic powders, Italian seasoning, salt and black pepper, mix them in, and cook for about 5 minutes on medium-high heat.
Add minced garlic to skillet, stir it in until fragrant, about 30 seconds.
Add in marinara sauce, stir it in well, bring to low boil then reduce heat to medium-low, and let simmer uncovered for about 8-10 minutes.
Lightly coat baking dish with oil spray, then lay zucchini halves skin down.
Fill each of them with the chicken marinara mixture. It's completely fine if they overflow in baking dish.
Sprinkle the shredded mozzarella on top, then add dish to oven for 23 to 28 minutes. After 23 min, carefully poke a couple of zucchini with tines of fork to see if they're cooked. Zucchini should feel tender, if they still feel tough, continue cooking for an additional 4-6 minutes.
Once cooked, carefully remove from oven, let cool for 2 to 3 minutes, then sprinkle chopped fresh basil all over. Serve warm as-is, over rice, with a side salad and enjoy!