While walking around the farmer's market in the San Francisco's Ferry Building, I started putting this together in my head. The only reason it's "for 4" is so that there are leftovers for the next day. It's for 2, and hopefully gets you a breakfast guest.
Set a roasting pan over two of the burners on your stove (I used the front and back, so that there was even heat under the pan), put your olive oil and butter in the pan, and set the two burners to medium-high heat. NOTE: The pan and its handles will get very hot, so be careful not to burn yourself.
While that's coming to temperature, rinse your pork tenderloin, and season it liberally with freshly cracked black pepper and some salt, rubbing it in on all sides.
Next, carefully place your tenderloin into the roasting pan. The reason for doing this is to sear the outside of the tenderloin so that when you roast it in the oven it stays nice and moist. So you'll want to sear it on all "sides" for about a minute each (rotating it with a pair of tongs), including the two ends.
Once you've seared the tenderloin, remove it from the pan and set it on a plate, and turn off the two burners. Add your chopped onion, white wine, and lemon juice to the pan, and stir it with a spoon. Return the tenderloin to the pan, and spoon some of the wine/juice/onions over the top. Now, using pot holders (as the pan is still hot), put the roasting pan into the oven.
This will cook for about an hour and a half, and the only thing you really need to do during that time is spoon the wine/juice/onions over the top of it a few times, about every 20 minutes or so, to keep it from drying out.
While it's happily roasting away, you'll want to get your Baby Spinach with White Beans and Pancetta and Twice Cooked Potatoes underway.
After about 45 minutes, you'll want to put your Twice Cooked Potatoes into the oven. They'll cook happily alongside the pork, and the timing will be just right.
After about an hour and fifteen minutes you can start checking on the tenderloin. Poking it with your finger should feel somewhat springy, and a sharp knife inserted to the middle should run clear juices. An hour and a half should give you a perfect, moist roast.
If you want to drop the meal into a holding pattern, you can once the hour and a half is up - just reduce the heat in the oven to around 200°, and you can keep it (and your Twice Cooked Potatoes) in the oven for another half hour.
Once you've taken the pork out of the oven, you'll want to let it rest for around five minutes. Once the five minutes are up, you can slice the pork into nice medallions.
To serve, put some of your Baby Spinach with White Beans and Pancetta on the plate, and lay two or three medallions of pork over it. Then place a couple of the Twice Cooked Potatoes along the side, and serve.
Click here to see some "behind-the-scenes" photos from this meal.
Enjoy!