To make brine mixture, boil 2 cups of water in a large saucepan over medium-high heat and add sugar and salt. Stir until completely dissolved. Add remaining cold water, vanilla, and spices, stirring to combine.
Place brine mixture in a large bowl and add tenderloins, making sure that the pork is completely submerged in brine. A heavy plate works well for this. Keep tenderloins in the brine for 6 to 8 hours, covered in the refrigerator. You can keep the pork in the brine for up to 14 hours. any longer and the texture of the meat can become rubbery. After brining pork, rinse completely in cold water twice, making sure ll spices are removed. Discard brining liquid and store tenderloins until ready to use.
Prepare plank for grilling. Coat tenderloins with olive oil and place them on a plank. Close lide and grill for 15 minutes. Turn off grill adn continue to cook meat with the grill lid closed for an additional 5 to 7 minutes. Using a meat thermometer, grill meat until cooked to an internal temperature between 145 and 150 degrees F for medium doneness.
Remove meat from grill and transfer to cutting board. Cover loosely with foil and allow meat to rest for an additional 5 minutes. The meat will continue to cook while resting. To serve, cut tenderloin into 1/2-inch slices and serve with warmed Blackberry Mustard Sauce.
To make the sauce, in a heavy-bottomed medium saucepan, melt 2 tablespoons butter over medium-high heat. Add garlic and cook until the garlic just begins to brown. Add wine, mustard, vinegar, and cumin and bring to a boil. Turn heat to medium and simmer until slightly reduced, about 10 to 12 minutes. Remove from heat and set aside.
Just before serving, reheat mustard sauce over medium heat, adding blackberry jam and remaining butter.
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