In a skillet, pour the olive oil on a medium heat then add the minced onion, garlic, salt, black pepper and red pepper flakes, stir from time to time until the onion softens ( You can go further and caramelized the onion if you like).
Add the spinach to the skillet in batches if you use a small skillet then stir until the spinach becomes wilted.
Spread the spinach mixture in an ovenproof baking tray, set aside.
In a medium-sized bowl, crack the eggs then add the heavy cream and whisk together until combined.
After that, add mozzarella and cheddar cheese to the eggs, stir well.
Pour the eggs over the spinach in the baking tray.
Bake at 180C for 25 minutes.
Garnish with some chopped parsley (optional) then serve immediately.