To make the white sauce first melt the butter in a saucepan. Add the flour and mustard and stir to form a paste. Over a gentle heat add the milk a little at a time until you have the consistency of pouring cream. Use a whisk and stir constantly to avoid any lumps.
Add a scratch of freshly grated nutmeg, the cheese and the onion to the sauce. Stir continuously and heat gently until the onion has softened and the sauce has thickened. Add a little extra milk if the sauce is too thick. Once cooked allow to stand for 30 mins off the heat before removing the onion.
Fry the bacon in a dry non-stick pan until crispy. Remove the bacon and set aside. Keep the bacon fat in the pan.
Season a little flour with salt and pepper and dust the fish, skin side only.
Fry the fish skin-side down in the bacon fat for 4 minutes, or until the skin is crispy.
Place the white sauce back on a gentle heat.Finely chop the parsley and stir through. Adjust the seasoning if necessary.
Flip the fish fillets over and cook for approx 30 seconds on the other side before removing from the heat.
Place the fish on warmed plates skin-side up. Spoon over the sauce and place the crispy bacon rashers on top. Serve immediately.