Pour everything into the prepared dish then place this into another ovenproof dish. Fill the outer dish with 2-3cm boiling water and carefully place the lot in the oven. Don't allow any water to splash into the souffle.
Preheat oven to 180 degrees C. Grease a round ovenproof dish with high sides.
Wash spinach and place into a large saucepan while still wet. Heat over a medium heat until it falls apart then drain in a sieve.
Melt butter in a small saucepan. Add onion and cook for 3-4 minutes until soft. Stir in flour then milk. Add salt, nutmeg and pepper then boil stirring continuously reduce heat and whisk to make a thick bechamel. Remove from heat.
Separate eggs. Whisk egg yolks in a large bowl until creamy then stir in the slightly cooled sauce.
Press the spinach to drain then cut finely. Add the spinach and parmesan to the mixture.
Whisk the egg white until stiff adding the tartar. Fold into the mixture.
Bake in preheated oven for 50-60 minutes or until brown then pierce with a knife if nothing sticks to it then it is done. Serve immediately.