I think I've finally cracked how to get a pork fillet perfectly cooker – not under, not dry and very tender.
It's all about heat and timing. First get your meat to room temperature – this is very important – if it is still fridge cold the middle will take a lot longer to cook and the outside will be dry.
I don't season the meat before searing it – but if you want to I suggest pepper or other spices only – no salt because salt will draw moisture out of the meat.
Get your heaviest fry pan hot – really hot – you might need your extractor fan on so you don't set off your smoke alarms. Then put in your oil (I use vegetable oil because it can take a lot of heat). Wait for it to be hot and then with a long pair of tongs put in your pork fillet – there should be a real sizzling sound – if there isn't, your pan is not hot enough. Watch out for oil spatters – they seem to go everywhere! Sear the meat on all sides – for no more than say 5 minutes in total.
Then put the meat into a roasting dish – it doesn't have to be hot – but you do need to have heated up your oven to a moderate heat. Add a splosh of white wine (or water) and cover the dish with tin foil. Cook for about 30 minutes or until your meat thermometer says it is almost done. Let the meat rest covered in the foil.
For the sauce I just used a store bought sachet of hollandaise sauce and kicked up by adding it to the pan juices from the pork and the wine, and also adding a teaspoon full of hot mustard powder, salt and pepper – stir it together. This sauce complemented the pork really well and got rave reviews from my better half.
An admission – I didn't think to take pictures at the time, so I used the leftovers the next morning to take the pic! Actually I found that a relaxing way to do it because I wasn't holding up dinner while faffing around with my pretty inadequate photo styling!