Trim away the woody base (1-2 cm) of the asparagus. Slice the asparagus into chunks of about 1 cm. Finely chop the onion and slice the mushrooms into quarters.
Heat 1 tablespoon of olive oil in a frying pan. Fry the mushrooms on a medium heat in about 7 minutes until cooked. Add the asparagus for the lad minute of cooking. Season with salt and pepper.
Heat the butter in a large saucepan and add the onion. Fry the onion on a low heat for about 5 minutes until it starts to soften. Add the flour, keep stirring well and simmer for 4 minutes on a low heat. Pour 250 ml of milk onto the flour mixture. Stir well until the sauce starts to bind and thicken. Gradually add the rest of the milk. Stir all the time to prevent lumps forming in the sauce. Stir to create a smooth sauce. Crumble the stock cubes above the sauce. .Add the mushrooms and asparagus. Mix well. Remove from the heat.
Grease the muffin tray with olive oil. Slice each sheet of filo pastry into 10 x 10 cm squares. Brush each square with olive oil. Push the squares of one sheet of filo pastry into a cavity in the muffin tray making sure they overlap. Repeat to fill three other cavities. Bake the filo pastry cases in the oven for about 8 minutes until crisp and golden. Remove them from the tray. Arrange on 4 plates and fill the cases with the asparagus and mushroom ragout.
