To begin, place the pork tenderloins in a gallon sized zip-lock bag.
Add the water, apple juice, garlic, kosher salt, and peppercorns. Remove as much of the air as possible and seal the bag closed. "Squish" the mixture around to combine and place the bag in a dish, in case of leaks. Refrigerate the pork in the brine for about 6 hours. You can do this in the morning and let it do its thing all day until it is time to make supper.
When it is time to make your pork tenderloin, heat your oven to 400 degrees.
Remove the tenderloins from the brine and pat the meat dry with paper towels.
In a small dish, combine the ground black pepper, onion powder, and garlic powder. Season the pork all over with the seasoning mixture.
Heat the avocado oil in a large cast iron skillet over medium high heat. Place the tenderloins in the skillet at cook undisturbed for about 3-4 minutes, or until the meat gets nicely browned. Turn it over and immediately slide the skillet into the oven.
Bake the pork tenderloin until a thermometer reaches 145 degrees. This will only take about 20-25 minutes.
Allow the meat to rest for about 5 minutes before slicing.