Preheat and Prepare: Preheat your oven to 180°C (160°C fan). Line a 33cm x 23cm (quarter) baking tray with non-stick baking parchment, ensuring there is a slight overhang on the sides to make removal easier. Drizzle the olive oil onto the parchment and spread it evenly.

Whisk the Eggs: In a large bowl, crack all 12 eggs. Add the whole milk, salt, pepper, and dried oregano. Whisk vigorously for a full minute. What works best for me is using a large balloon whisk; this incorporates plenty of air, which is the secret to a fluffy result.

Pour into the Pan: Gently pour the egg mixture into the prepared baking tray.

Add the Toppings: Evenly scatter the fresh spinach, halved cherry tomatoes, and diced red onion over the surface of the egg mixture.

Add the Cheese: Crumble the feta cheese and distribute it evenly over the top of the vegetables.

Bake to Perfection: Place the tray in the preheated oven and bake for 12-15 minutes.

Check for Doneness: The eggs are ready when the centre is just set (it shouldn't be liquidy) and the edges are lightly golden. If you gently shake the pan, there should be a very slight wobble in the very centre. This ensures it won't be overcooked.

Rest and Garnish: Remove from the oven and let it cool in the tray for a few minutes. This allows it to set fully, making it easier to slice. Garnish with a sprinkle of freshly chopped parsley.

Serve: Use the parchment paper overhangs to lift the baked eggs out of the pan and onto a cutting board. Slice into squares and serve immediately.