Butter the mold and sprinkle with cheese. Preheat oven to 400 degrees. Measure out your ingredients.
Cook the shallots or onions for a moment in the butter. Add spinach and salt, and stir over moderately high heat for several minutes to evaporate as much moisture as possible from the spinach. Remove from heat.
Prepare the soufflé sauce base. After the egg yolks have been beaten in, stir in the spinach. Correct seasoning.
Beat the egg whites and salt until stiff. Stir one fourth of them into the sauce. Stir in all but a tablespoon of the cheese. Fold in the rest of the egg whites and turn mixture into prepared mold. Sprinkle with remaining cheese and set on a rack in the middle level of preheated oven. Turn heat down to 375 degrees and bake for 25 to 30 minutes.