Butter ramekins and sprinkle with a little Parmesan.
Rinse and spin dry spinach. Cook in a pan with a little boiling salted water. Drain, squeeze well and chop finely.
Melt butter in a saucepan, add flour and cook briefly. Add milk while stirring and simmer until thickens, for about 2 minutes. If sauce is too thick, add more milk and simmer for 1-2 minutes. Remove from heat.
Separate eggs. Whisk egg yolks and stir into sauce. Add remaining Parmesan and spinach. Beat egg whites until stiff. Fold in batter, add walnuts.
Pour soufflé batter into prepared ramekins and bake in preheated oven at 190°C (approximately 375°C) for about 20-25 minutes. Remove from oven and serve warm.