Prepare Zucchini: Preheat your oven to 375°F (190°C). Slice zucchinis in half lengthwise and scoop out the center using a spoon, leaving about ½-inch of the flesh intact to create boats. Place zucchini halves on a baking sheet lined with parchment paper.

Cook Vegetables: Heat olive oil in a large skillet over medium heat. Add chopped mushrooms and sauté for 5 minutes until softened. Add garlic, spinach, oregano, salt, and pepper. Cook until the spinach wilts, about 2 minutes. Remove from heat and let cool slightly.

Make Filling: In a mixing bowl, combine ricotta cheese, mozzarella, Parmesan, and the cooked vegetable mixture. Stir until well combined.

Stuff Zucchini: Spoon the filling into each zucchini boat, pressing it down lightly to pack it in.

Bake: Bake in the preheated oven for 20-25 minutes, or until the zucchini is tender and the filling is golden and bubbly.

Serve: Garnish with fresh basil, if desired, and serve hot.