First of all, trim the excess fat and sinew from the pork tenderloins. Then, in a small bowl, beat the egg together with the 2 mustards and pour it into an oblong dish, which is at least 1 inch (2.5 cm) deep and as long as the pork tenderloin.
Next, chop the garlic and parsley (in a mini-chopper or processor, if you have one), add the breadcrumbs and a seasoning of salt and pepper, whiz for a couple of seconds then spread them out on to another oblong dish (same size as above).
Now roll the pork fillets in the egg mixture first, until they are very well coated. Then lift them out of the egg mixture, letting the excess egg drip back into the dish, then lay them in the crumb mixture. Turn the fillets over in the crumbs until they are evenly coated and there are almost no loose crumbs left in the dish.
After that, transfer them to the greased baking tray and roast the fillets in the pre-heated oven for 30 minutes. This will give you pork that is still slightly pink in the middle. If you prefer it more well done, cook it for a further 10 minutes. (If you are preparing the pork a couple of hours in advance, just cover it with clingfilm and keep it in the fridge till needed but allow it to come back to room temperature before cooking.)
Meanwhile, for the sauce, heat the butter in a small saucepan and cook the onion in it on a medium heat for about 10 minutes until softened and golden brown.
Then add the white wine, turn up the heat and boil for 10 minutes to reduce slightly, before adding the crème fraîche and mustards, then simmer gently for a further 10 minutes.
When the pork is cooked, let it rest in a warm place for about 10 minutes before carving into slices and serving with the sauce.