Put the spinach in a large saucepan.
Pour over enough water just to cover the spinach.
Add 1 teaspoon of the salt and place the pan over moderately high heat.
Bring the water to the boil.
Reduce the heat to moderate and cook the spinach for 10 minutes.
Drain the spinach in a colander, pressing it well down with a wooden spoon to remove all excess liquid.
In a large frying pan, melt the butter over moderate heat.
When the foam subsides, add the spinach to the pan.
Gradually stir in three quarters of the cheese with a wooden spoon and cook, still stirring, for 3 minutes, or until the ingredients are well blended.
Add the pepper and remaining salt.
Preheat the grill [broiler] to high.
Remove the pan from the heat and transfer the spinach mixture to a flameproof casserole.
Sprinkle the remaining cheese on top of the spinach and place it under the grill [broiler].
Cook for 7 to 10 minutes, or until the cheese is golden and melted.
Remove the casserole from the heat and serve immediately.