Preheat the oven to 375°F (190°C) and line a baking sheet with parchment paper or grease lightly.
Slice the zucchini lengthwise and scoop out the seeds to form a “boat,” leaving a ¼-inch border. Arrange on the sheet, drizzle with olive oil, and season with salt and pepper.
Heat olive oil in a skillet over medium heat.
Add garlic and sauté for 30 seconds. Cook mushrooms for 3–4 minutes until softened.
Add spinach and cook until wilted (2–3 minutes). Stir in chopped basil and let cool slightly.
In a bowl, mix the spinach-mushroom mixture with ricotta until combined.
Fill each zucchini half with the ricotta mixture. Sprinkle mozzarella evenly over the tops.
Bake for 20–25 minutes until the zucchini is tender and cheese is golden. Cool slightly, garnish with fresh basil if desired, and serve.