Pork Tenderloin with Port Sauce

Source: "James" via Martha Stewart via my mother

Note, this recipe isn't on her site anymore, so I can't link it.

This is my second favorite way to prepare pork tenderloin. My very favorite is pork with balsamic vinegar from Marianne Esposito, but this recipe is much quicker and really really good. It's a great weeknight dinner. Best of all, I almost always have the ingredients on hand!

Ingredients
2 Tbl olive (or vegetable oil)
2 pork tenderloins - (1 lb ea)
Coarse salt -, To Taste
Freshly-ground black pepper, to taste
1/2 cup port wine
1 cup chicken stock
2 Tbl butter - (1/4 stick), Unsalted
1 Tsp red-wine vinegar, or more to taste

Instructions
1. Heat oil in a medium, heavy-bottomed skillet over high heat.
2. Remove silver skin from pork tenderloins, pat dry, season with salt and pepper and slice crosswise into 1-inch pieces.
3. Brown medallions in oil. Transfer to a warm platter; set aside.
4. Remove skillet from heat. Discard fat, and add port; return skillet to heat. Deglaze sauce, scraping with a wooden spoon to loosen the browned bits from bottom of pan. Continue cooking until sauce is reduced to about 2 tablespoons.
5. Add broth, and cook until sauce is lightly syrupy, about 4 minutes.
6. Whisk in butter, a little at a time, gently shaking pan while whisking. Stir in vinegar. Taste, and adjust for seasoning. Spoon sauce over pork, and serve immediately.