Pat the pork loin dry with paper towels and set it aside.
In a small bowl, mix together the brown sugar, dried sage, garlic salt (if you only have garlic powder just add an additional half teaspoon of salt), and sea salt. Sprinkle the dry rub all over the pork loin and coat evenly. Do not forget the ends.
In a large skillet, heat 2 tablespoons unsalted butter on medium heat and sear all sides. It is important to allow the rub to brown but not burn to lock in the flavor. Place the pork loin into the slow cooker.
In a small pot melt 1/2 cup butter over medium-low heat. Add the minced garlic and stir for about 1 minute, until fragrant.
Add the honey, fresh sage, salt, and pepper and allow to simmer for 2 minutes while stirring frequently. Pour the sauce over the pork, and cover with the lid.
Cook on LOW for 3.5 hours, rotating halfway through the cooking time. Depending on the size of your pork loin cook time may vary slightly. It's always best to check the internal temperature with a meat thermometer. (155 degrees F for medium-well or 160 degrees F for well done.)
Transfer the pork loin to a platter or cutting board, and tent with tin foil to rest.
Pour the juices from the slow cooker into a small pot, reserving 1/4 cup of the juices.
Add the cornstarch to the reserved bowl and mix until dissolved. Add water and stir once more to finish the slurry. Pour the slurry into the pot and bring to simmer over medium-low heat until thickened.
Slice the pork loin and drizzled the pan sauce on top for serving.