Preheat oven to 200°C (400°F). Spray olive oil into a 1.5-liter glass or ceramic baking dish. Set aside.
Prepare the egg batter
In a large mixing bowl, whisk together eggs, heavy cream, shredded parmesan, garlic powder, salt, dried oregano, salt, and pepper. I used a hand whisk, and this process took me 1 minute, don't over whisk. Simply whisk enough to break eggs and combine with milk and spices. Set aside in the fridge.
Saute the vegetables
Under medium heat, warm olive oil into a large non-stick pan.
Stir in the diced onion and cook for 1- 2 minutes until fragrant and transparent.
Stir in diced capsicum and sliced mushrooms. Sauté the vegetables for 2-3 minutes until fragrant and mushrooms have released their water.
Pile the chopped spinach in the pan, cover with a lid, reduce to low heat, and cook, stirring every 30 seconds until the spinach leaves wilt. It will significantly decrease in volume as spinach leaves will release their water.
Remove lid, stir and cook for an extra 1-2 minutes to evaporate the water from the spinach. You don't want to add water to the frittata, or it won't firm up in the oven. Note: If your pan is too small, you can add the spinach in two parts, repeat the cooking process two times.
Use a slotted spoon to transfer the sautéed vegetables into a 9-inch x 9-inch baking dish. This avoids transferring any remaining vegetable juice from the pan.
Pour the previous egg mixture on top of the cooked vegetables.
Add shredded cheddar and crumble some goat cheese all over the top of the pan
Bake in the oven's middle rack for 30-35 minutes or until the filling is set. Check if cooked through by cutting a tiny bit in the center of the frittata. If not set, bake longer.
Serve hot with a green salad on the side.
Store in the fridge in the baking dish for up to 3 days or freeze. Rewarm in the oven or microwave
You can freeze frittata slices in a sealed container, thaw in the fridge the day before.
Frozen spinach can be used, and you will need 10.5 oz.Mushrooms: you can replace mushrooms with bacon pieces if not vegetarian.