1 cup hard apple cider, such as Woodchuck Amber or Granny Smith
3 tablespoons kosher salt
3 tablespoons brown sugar
1 cinnamon stick
1 star anise pod, or ½ teaspoon anise seed or fennel seed
½ teaspoon allspice berries
½ teaspoon black peppercorns
½ teaspoon coriander seeds
½ teaspoon mustard seeds
1 large or 2 small bay leaves
3 sprigs of fresh thyme, briefly rolled between hands to release oils
1 pork tenderloin, about 1 pound
2 tablespoons extra-virgin olive oil
Combine the cider, salt, and brown sugar in a medium glass or stainless steel bowl with 2 cups lukewarm water. Whisk until the salt and sugar are completely dissolved.
Gently crush the cinnamon stick, star anise, allspice berries, peppercorns, coriander seeds, and mustard seeds with a mortar and pestle until fragrant and crumbly. (You can also put them in a Ziploc bag and crush them with a rolling pin or the bottom of a heavy skillet, or you can pulse them 2 or 3 times in a spice grinder.) Add the spices to the brine, along with the bay leaves and thyme, and stir gently to distribute evenly.
Submerge the tenderloin in the brine, cover the bowl with plastic wrap, and refrigerate for at least 8 hours, preferably overnight.
Heat the oven to 400°F, and line a rimmed baking sheet with parchment paper. Remove the tenderloin from the brine, pick off and discard any spice shards, and pat completely dry with paper towels. Allow to return to room temperature, about 15 minutes.
Put the olive oil in a large skillet over medium-high heat. When it’s hot, add the tenderloin and cook, turning every 3 to 5 minutes, until browned on all sides, about 15 minutes total.
Transfer the tenderloin to the baking sheet, and roast until its juices run clear and/or a meat thermometer inserted into the thickest point reads 140°F, 15 to 20 minutes. Remove the tenderloin from the oven, cover it loosely with foil, and let it rest for 10 minutes. Cut into medallions on a bias, and serve. (Store leftover tenderloin wrapped in foil or plastic wrap in the refrigerator for up to a few days.)