Warm a little olive oil in a frying pan, add the spinach and cook over a medium heat for about 7 minutes, stirring all the while.
Make the Béchamel sauce: add the flour and walnut drink to the frying pan and gently stir for a couple of minutes, until you have a smooth mixture binding all the ingredients together. Add salt, pepper and nutmeg to taste. Keep stirring to ensure the mixture remains smooth and creamy, without any lumps. The flour needs to cook completely for a thick, tasty sauce.
Once the sauce has the desired texture, pour the mixture into an oven dish, sprinkle the grated cheese on top and bake au gratin for about 5 minutes in a preheated oven at 180 ºC, until the melted cheese turns golden brown and crispy. Ready to serve!