To make the pancakes, put the flour into a large bowl and add the egg and half the milk. Mix well with a hand mixer until it is bubbly. Stir in the rest of the milk.
Heat an 18–20cm (7–8in) shallow frying pan, add a few drops of oil and, when hot, pour in enough batter, tilting the pan to coat the base evenly. Cook until the pancake browns underneath, then turn it over using a palette knife and cook for another half a minute. Slide the pancake out onto a plate. Make 8 pancakes, separating each with kitchen paper. Set aside, keeping warm
Then make the filling. Heat a frying pan and fry the bacon and onion for 5 minutes. Add the mushrooms, cook for 5 minutes and season well. Preheat the oven to 200°C/400°F/Gas 6.
To make the sauce, melt the butter in a small saucepan, add the flour and cook the paste for a minute.
Gradually beat in the milk. Bring to the boil, stirring for a thick smooth sauce. Stir in the mustard and half the cheese. Then add half of the sauce to the mushroom mixture together with the chopped parsley.
Place 1 pancake on a buttered ovenproof plate or baking sheet. Spread with some of the mushroom filling, put another pancake on top and continue spreading and layering, ending with a pancake. Sprinkle the rest of the cheese on top. Cover loosely with foil.
Bake for 10 minutes to heat through, then remove the foil to brown the cheese a little for about 5 minutes.
Reheat the remaining sauce, adding a little more milk or water to thin it down if you like. Cut the pancake stack into wedges, pour over the sauce and serve with heaps of freshly cooked vegetables.