Preheat your oven to 180°C (350°F) and grease a baking dish with a light coating of oil or butter.
In a large bowl, combine the grated zucchini, chopped celery, sliced mushrooms, peas, parsley, and green onions.
Add the grated cheese and oatmeal to the vegetable mixture and mix well.
Beat the eggs in a separate bowl and then pour them into the vegetable mixture. Stir everything together until it's evenly combined.
Season with salt and pepper to taste.
Transfer the mixture into your prepared baking dish and spread it out evenly.
Bake for 30-35 minutes, or until the top is golden and the center is set.
Remove from the oven and let it cool for a few minutes before slicing and serving.