As you stated, there are so many options. How about this idea; Cut the slit in the tenderloin and place the whole thing in milk overnight. The next day, make a dressing/stuffing of chopped bamboo shoots, water chestnuts, bok choy, and coarsly chopped onion. You could even incorporate mushrooms and brocolli into this. Steam or stir-fry the veggies. Season the veggies with with soy sauce, a touch of sesame oil, Chinese 5-spice powder, granulated garlic, and ginger. Let that sit for about ten minutes to allow the flavors to mingle. Oh, and the veggies should be hot, but still very crisp.
Rub the tenderloin inside and out with a mixture of soy sauce, onion powder, garlic powder, ginger, and a little brown sugar or honey. Stuff the tenderloin with the veggies, and place the stuffed roast onto a large piece of heavy-duty foil. Tie the roast closed with butcher's twine. Pour any remaining liquid over the roast and seal teh foil around it. Place the roast into a hot 425' oven for 15 minutes. Turn the temperature down to 300, and roast for 40 minutes more.
When the time is up, test with a meat thermometer. The meat should read about 150 or so (though some like their pork at 145' F.)
Serve with a fragrant jasmine rice. Use the meat juices as a sauce to dress the rice.
Another option goes in a completely different direction. Make a dry rub out of brown sugar, onion, garlic, chili powder, and Worcestershire sauce. Rub the outside of the meat completely. Cut a slit into the pork tenderloin and stuff with a good Andouile, or chorizo type of sausage, and mushrooms. Cover in foil and bake low and slow for a couple hours. Serve with fresh, corn tortilla chips, hot from the stove, and salsa.
Option 3: season with salt, pepper, and Thyme, and maybe a little rosemary. Sear the entire loin on all sides. Roast in a 350' oven until the meat thermometer reads 145'F. Serve with steak potato cubes.
Steak potato cubes: Use very large rusett potatoes. Wash and cut the potatoes into three slices along the lenth of the potato. Turn them on the uncut side and repeat the three slices. Now cut the slices into bite-sized cubes (about a half-ich cube). Boil until cooked through. Drain in a collander for a minute or so. Transfer teh potatoes to 3 inches of 165' cooking oil and brown all sides. Remove to paper towels to drain. Salt to taste. Serve with the pork tenderloin and some steamed greens such as beet or collard greens.
Pineapple sweet & sour sauce would go well with the first and last option.
You can also stuff the pork with fruit, such as apple, cherries, pineapple, peach, apricot, etc.
Other stuffing options might include a rice pilaf, or cornbread stuffing, or breadfast sausage mixed with a favorite barbecue sauce such as honey-mustard, or a Carolina style vinagar barbecue sauce. I could go on, but it's time to go home.
Seeeeeya; Chief Longwind of the North