In a large skillet, heat 2 tablespoons of olive oil over medium heat. Add 1 diced yellow onion and sauté until soft, around 5 minutes. Mix in 2 chopped spring onions, cooking until golden, about another 3 minutes. Stir in 2 cups of frozen chopped spinach and ¼ cup of chopped fresh parsley, sauté until moisture evaporates, around 5–7 minutes.

In a large mixing bowl, blend the drained spinach mixture with 8 ounces of cream cheese, ½ cup of extra sharp cheddar cheese, and ¼ cup of grated Parmesan cheese. Season with a pinch of ground nutmeg, white pepper, and sea salt, mixing thoroughly.

Preheat your oven to 400°F (200°C). Place a baking sheet on the middle rack.

On a lightly floured surface, roll out 1 sheet of puff pastry to fit a 9-inch tart pan, about ⅛ inch thick.

Spoon the spinach and cheese filling into the lined tart pan, smoothing it out evenly. Create small wells for the eggs.

Crack 4 medium eggs into the prepared wells. Roll out a second sheet of puff pastry to cover completely, sealing well and cutting small slits for steam.

Bake in the preheated oven on the baking sheet for about 35 minutes, until golden brown and puffed up.

Allow to cool for at least 30 minutes before slicing.