Heat the oven to 200C/Gas 6. Pierce the bag of spinach in several places and microwave on full power for 1 1/2 - 2 minutes until wilted. Leave until cool enough to handle then squeeze out as much liquid as possible. Chop roughly.
Melt the butter in a pan and stir in the flour. Cook stirring for 2 minutes. Gradually whisk in the milk until smooth then cook gently, stirring for 5 minutes. Transfer to a large bowl and set aside to cool slightly.
Brush 6-8 ramekins or small ovenproof dishes with melted butter and coat with breadcrumbs.
Stir most of the cheese and the mustard into the sauce and season with nutmeg, salt and pepper. Gradually stir in the egg yolks, then add the spinach.
In a large bowl, whisk the egg whites until forming soft peaks. Take care not to allow them to become dry. Spoon two tablespoons of egg white into the cheese mixture to slacken the mixture then fold in the rest of the egg white with a metal spoon.
Place the ramekins in a roasting tin and pour in boiling water to come half way up the sides of the dishes. Bake for 10-12 minutes until well risen and golden brown, but still with a little bit of a wobble!