Prepare the Zucchini:
Preheat the oven to 375°F (190°C). Cut the zucchini in half lengthwise and scoop out the seeds using a spoon, creating a hollow center. Place the zucchini halves on a baking sheet lined with parchment paper, drizzle with a little olive oil, and season with salt and pepper. Set aside.

Cook the Filling:
In a large skillet, heat olive oil over medium heat. Add the chopped onion and garlic, sautéing for 2-3 minutes until softened.
Add the chopped mushrooms and cook for another 4-5 minutes until the mushrooms release their moisture and become tender.
Stir in the chopped spinach and cook for an additional 2 minutes, just until the spinach wilts.
Remove the skillet from heat and let the mixture cool slightly.

Stuff the Zucchini:
In a bowl, combine the ricotta cheese, Parmesan cheese, oregano, salt, and pepper. Once the veggie mixture has cooled, fold it into the ricotta mixture until well combined.
Spoon the mixture evenly into the hollowed-out zucchini halves, pressing it down slightly to pack the filling.

Bake the Zucchini:
Place the stuffed zucchini on a baking sheet and bake in the preheated oven for 20-25 minutes, or until the zucchini is tender and the filling is golden brown on top.
If desired, sprinkle the mozzarella cheese over the top of the stuffed zucchini during the last 5 minutes of baking and return to the oven to melt.

Serve:
Remove the zucchini from the oven and garnish with fresh parsley before serving. Serve warm.