Preheat the oven to 400 degrees F (200 degrees C).
With 2 or 3 bits of string, tie up your pork loin, do this any way you like. It doesn't have to be fussy, you just want to keep the meat in a snug shape while it's cooking. Season generously with salt and pepper, then roll the meat in the fennel seeds until covered.
In a casserole pan or roasting tray, fry the meat for a couple of minutes in half the butter and a little olive oil, until nice and golden.
Throw in the garlic, herbs, fennel, and wine, then cover the tray loosely with some wet greaseproof paper and cook until an inserted meat thermometer reaches 150 degrees F. As the pork loin is off the bone it cooks very quickly. Remove from the oven and allow the meat to rest on a plate. Then, without using any more heat, finish off your sauce in the pan, scraping any goodness off the bottom and adding the rest of the butter. Remove any large bits.