Add the water, brown sugar, maple syrup, salt, apple cider vinegar, sage, rosemary, peppercorns, and mustard seeds to a large pot. Cook over medium high heat until the mixture begins to boil. Once the mixture has been boiling for a few minutes and the sugar and salt are completely dissolved, remove from heat and let cool completely. If you want the brine to cool more quickly, you can stick it in the fridge. I would not recommend cooling it with ice cubes as it may dilute the flavor.
Add the brine and pork to a large marinating bag or a large bowl. You want the pork to be completely submerged. Transfer to a refrigerator and let the tenderloin sit in the brine for 1-2 hours.
While the tenderloin is brining, make the maple glaze. Mix together 3 Tbs of each, maple syrup, dijon mustard, and apple cider vinegar. Set aside.
Preheat your oven to 350 °F (177 °C) for the last 20 minutes of the brine.
Once the pork is done brining, rinse with water and pat dry.
Heat oil in a cast iron skillet over medium high heat. Add the pork to the skillet and brown the meat for 2 minutes on each side. Brush the maple glaze over the browned pork tenderloin. Reserve any leftover glaze to use for the pan sauce.
Transfer the pork tenderloin to the oven to finish cooking. Roast for 25 minutes or until the tenderloin reaches an internal temperature of 145 °F (63 °C)
Remove from the oven and transfer the tenderloin to a cutting board to rest for at least 10 minutes.
While the pork is resting, make the pan sauce. Add the remaining maple glaze, 1 tablespoon of butter, and 2 sage leaves to the skillet with the pork’s rendered juice. Let the butter melt and stir the ingredients to create a decadent pan sauce. Spoon this over the pork when you are ready to serve.
Notes
You do not need to season the pork before brining.You do not need to cook the pork right after you brine it. In fact, letting the meat air dry will help it brown, but is not necessary.You can keep the unused brine in a sealed container for up to 1 week in the refrigerator.Do not reuse the brine after it has been in contact with raw meat.Do not brine the pork tenderloin over 2 hours or it may become gummy.If making this recipe with pork chops instead, brine for 30 minutes.