Preheat the oven to 190C and butter a soufflé dish.
Make a béchamel with two-thirds of the butter, flour and MEVGAL Milk and stir on the heat until it thickens. Add the salt, pepper and mustard powder and stir. Allow the béchamel to cool for 10 minutes.
Blanch the spinach for 2–3 minutes in salted water and then drain well and chop. Heat the rest of the butter in a frying pan and sauté the spinach with the spring onions until any liquid has evaporated. Beat the egg yolks and add the MEVGAL Kefalograviera Cheese. Add this mixture to the béchamel and then add the spinach and spring onions and stir well.
Beat the egg whites with a little salt into a tight meringue and gently fold into the béchamel mixture. Put this mixture into the soufflé dish and bake for 35–40 minutes until it rises. Serve immediately.