Preheat oven to 180°C (350°F). Grease a medium baking dish.
Rinse quinoa thoroughly and cook with vegetable broth in a saucepan over medium heat (about 15 minutes or until liquid is absorbed). Set aside.
In a skillet, heat olive oil and sauté onion and garlic for 2 minutes.
Add zucchini, red pepper, and cherry tomatoes. Cook until veggies are just tender.
Stir in spinach until wilted. Remove from heat.
In a large bowl, combine cooked quinoa, sautéed veggies, oregano, salt, pepper, beaten eggs, and 1 cup mozzarella. Mix well.
Pour mixture into prepared baking dish. Top with remaining cheese and Parmesan if using.
Bake for 25–30 minutes until golden and bubbly.
Let rest for 10 minutes before slicing. Serve warm or at room temperature.