I found some pork tenderloins on special a few weeks ago, so I bought a few, made some into freezer meals and left the rest "plain." I'm thawing the last one out today for dinner. Here's the marinade I'm trying, using fresh flat-leaf parsley from our garden - tried to make it easy with the measurements, but you can tweak here and there to your liking. We're going to grill it, but you could prepare it in the oven, too.
Marinated Pork Tenderloin for the Grill
1⁄3 c. olive oil
1⁄3 c. soy sauce
1⁄3 c. c. red wine vinegar
Juice of 1 lemon (about 2-3 tbsp.)
2 tbsp Worcestershire sauce
4 cloves garlic, minced
2 tbsp fresh parsley, finely chopped
2 tsp. dry mustard
1/2 tsp. freshly ground black pepper
1 1-lb. pork tenderloin
Whisk together all ingredients, and either pour over tenderloin in a non-reactive dish, or put it all in a Ziploc bag and marinate in the fridge for at least 3-4 hours. You could toss it in the freezer for another night, too (or even better, make one for tonight, and another for the freezer!) - just don't thaw it out and then re-freeze it without cooking the meat.
Preheat your grill, and then grill tenderloin until meat reaches 160 degrees. Toss some veggies or potatoes in a grill pan to do the whole meal on the grill.
To prepare in the oven, preheat oven to 350 degrees. If you're feeling fancy, you can sear the pork in a skillet with a
teeny bit of oil before you put it in the oven. It adds a little flavor
and helps keep it moist. Place in the oven and cook for 30-40 minutes or until the meat has reached 160 degrees. Let rest for at least 5 minutes before slicing.