In a medium saucepan, heat the butter over medium-low heat until melted. Add the flour and stir until smooth. Over medium heat, cook, until the mixture turns a light, golden sandy color, about 5 minutes.
Add the milk to the mixture 1 cup at a time, whisking continuously until very smooth. Bring to a boil. Cook 10 – 20 minutes, stirring constantly, then add the manchego cheese and continue to stir until all melted. Season with salt and nutmeg, and set aside until ready to use.
How to Make the Empanada Filling:
Sweat leeks and minced garlic in butter and oil for 10 minutes then set aside. Roast mushrooms on a baking tray in oven at 225 degrees C ( 437 ℉) for about 10 minutes with butter, salt and pepper.
Meanwhile cook out diced silver beet until soft, add chopped green leaves until wilted. Add the leek and garlic mixture, and then the roasted mushrooms chopped roughly.
Drain excess liquid, if any. Stir in Bechamel sauce and add salt and pepper to taste. See Empanada Dough recipe for assembly and frying methods.