Put the spinach in a pan with ⅝ cup (1.5 dl) of water, 1 tablespoon (8 g) of butter, and half the salt. Bring briskly to the boil. Remove the pan from the heat, drain the spinach and press out all the juice. Chop the spinach very finely.
Put the chopped spinach in a pan with the remaining butter, stir in the flour and the cream, the whole onion, the rest of the salt and a little pepper and nutmeg. Stir over low heat for 10 minutes. Remove the onion, stir in the lemon juice and serve, garnished with the hard-boiled eggs.