Preheat oven to 375°F. Lightly grease a baking dish. Cut zucchini in half lengthwise, scoop out the center to make boats, and sprinkle with salt. Set aside.
In a skillet, heat olive oil over medium. Sauté onion for 3–4 minutes. Add garlic and mushrooms, cook 5 minutes.
Stir in spinach, seasoning, and red pepper flakes. Cook until spinach wilts, about 2–3 minutes.
Transfer cooked veggies to a bowl. Mix in ricotta, Parmesan, salt, and pepper.
Pat zucchini dry. Fill each with the ricotta mixture. Sprinkle mozzarella on top if using.
Bake uncovered for 20–25 minutes, until zucchini is soft and cheese is golden.
Top with fresh basil or parsley and serve warm.