Line a baking tray with greaseproof paper and layer the bread on top.
Grease a baking dish with 1 tablespoon of butter and set aside.
Cook the bread in the oven for 10 minutes until lightly toasted. Allow to cool.
Melt 1 tablespoon of butter in a large pan over medium heat.
Add the onion and sauté until soft, about 5 minutes.
Add ½ teaspoon of salt, some pepper and continue cooking for one minute.
Stir in the thawed spinach, remove from heat and set aside.
Spread half of the bread cubes in the dish and top with half of the spinach mixture and half of cheeses.
Repeat layering with remaining bread, spinach and cheeses.
Whisk the eggs, milk, nutmeg, and remaining ½ teaspoon of salt and another dash of pepper in a large bowl and pour over the casserole, making sure to cover it evenly.
Cover the casserole with plastic wrap and refrigerate overnight.
Allow the casserole to come to room temperature, about 30-45 minutes.
Preheat the oven to 120°C / 250°F / Gas mark ½.
Bake uncovered in the middle of the oven until golden brown and cooked through, approximately 50 minutes.
To get it extra crispy on top, place under the grill on high for 1-2 minutes. Let stand 5 minutes before serving.