Mix water, salt, 1/3 cup maple syrup, garlic, ginger, rosemary, black pepper, and red pepper flakes in a large bowl. Place pork loin in brine mixture and refrigerate for 8 to 10 hours.
Remove pork from brine, pat dry, and season all sides with salt and black pepper.
Preheat oven to 325 degrees F (165 degrees C).
Heat vegetable oil in an oven-proof skillet over high heat. Cook pork, turning to brown each side, about 10 minutes total.
Transfer skillet to the oven and roast until pork is browned, about 40 minutes.
Mix 2 tablespoons maple syrup and Dijon mustard together in a small bowl.
Remove pork roast from the oven and spread maple syrup mixture on all sides. Cook for an additional 15 minutes, until the pork is no longer pink in the center. An instant-read thermometer inserted into the center should read 145 degrees F (63 degrees C).