Cut the zucchinis in half lengthwise and scoop out the centre flesh with a spoon, leaving about 1 cm (½ inch) of shell. Reserve a little of the scooped-out flesh.
Place the zucchini halves on a lightly greased baking dish, cut side up. Brush with a little olive oil and sprinkle lightly with salt.
Cook the Filling
Heat 1 tbsp olive oil in a large pan over medium heat.
Sauté the onion and garlic until fragrant (about 2–3 minutes).
Add the mushrooms and reserved zucchini flesh. Cook until softened and any moisture has evaporated (about 5 minutes).
Stir in the spinach and cook until wilted (about 2 minutes). Remove from heat and let cool slightly.
Mix the Ricotta Filling
In a bowl, combine the ricotta, Parmesan, and (if using) the egg.
Add the cooked spinach-mushroom mixture, oregano, salt, and black pepper. Mix until well combined.
Stuff the Zucchini Boats
Spoon the ricotta-spinach-mushroom mixture evenly into the zucchini halves.
Sprinkle mozzarella cheese and a little extra Parmesan on top.
Bake
Bake for 20–25 minutes, or until the zucchinis are tender and the cheese is golden and bubbly.
Serve
Allow to cool for a few minutes before serving.
Serve warm as a main or side dish.
Tips
Add a pinch of nutmeg to the ricotta mixture for extra flavour.
For a non-vegetarian version, stir in cooked chicken or turkey mince to the filling.