Pre-heat the oven to 350°F.
Slice only the very top of the ciabatta bread off and scoop out the filling. Place the bread onto a baking sheet and brush the inside with olive oil and toast in the oven for 8 minutes, just to crip up the bread slightly.
While the bread is toasting, add the eggs, milk and salt and pepper into a large bowl and whisk until well combined. Stir in the spinach, feta and lemon zest
Remove the bread from the oven and allow it to cool slightly before filling with the egg mixture. Place the bread back into the oven and bake for 30 minutes, or until the eggs are just set. Do not over cook. Note: the eggs will continue to cook slightly after you remove it from the oven. Cut into slices and serve warm.