Brine: In a gallon-size freezer bag (I prop it up in a larger bowl to make it easier to work with), mix kosher salt with warm water until dissolved. Whisk in vinegar, brown sugar and ice, then add the pork (make sure pork is submerged). Brine for exactly 20 minutes (meanwhile see steps below). Remove pork from brine, RINSE thoroughly and pat very dry. The pork can become mealy if left in the brine any longer than 20 minutes.
Spice Mix and Sauce: While the pork is brining, whisk together all of the Spice Mix ingredients in a small bowl; set aside. In a separate small bowl, whisk together all of the Chili Dijon Sauce ingredients. Preheat oven to 400 degrees F.
Spice Rub: Evenly rub the dried pork all over with the Spice Mix. Tuck the thin end of the fillet under so the pork is roughly the same thickness from end to end, and so it fits in a cast iron skillet (see photos in post or watch video if needed).
Sear Pork: Heat 2 tablespoons vegetable oil in a large cast iron skillet over medium-high heat. When the oil is very hot and just smoking, add pork. Sear each side until golden, then transfer to a plate. Once the skillet is cool enough to handle, wipe clean. Meanwhile…
Prep bacon: Lay the bacon strips vertically on a cutting board, slightly overlapping (see photo in post). There should be enough bacon so when rolled, it will wrap the length of the pork.
Wrap pork in bacon: When the pork is cool enough to handle, place it at the end of the arranged bacon, closest to you. At this point, double check you have the correct width of bacon, and adjust as needed. Slip a long knife under the bacon and pork tenderloin and use it to help you lift the bacon and roll the pork so the bacon wraps around the fillet.
Optional - Trim bacon if desired: Only the bacon on the outside of the pork tenderloin will become crispy, not the bacon that is overlapped. So, I suggest trimming the length of the bacon after wrapping so it only overlaps by 1 ½ inches or so. The bacon will shrink while it cooks, and you’ll be left with only crispy bacon!
Glaze: Transfer tenderloin back to the wiped-out skillet seam side DOWN. Separate out 2 tablespoons Chili Dijon Sauce and brush it evenly over the bacon wrapped pork.
Bake: Bake at 400 degrees F until the pork registers about 138 degrees F on an instant read thermometer, about 20 minutes depending on thickness. (Note: this recipe takes much longer to cook than traditional pork tenderloin due to the bacon).
Baste: Remove from the oven and baste/brush the bacon with the pooled pan juices, as well as some additional reserved Chili Dijon Sauce that never touched the raw pork (this is what will make the bacon deeply golden).
Broil: Return to the oven and turn the broiler on high and broil until bacon is crispy, taking care that the bacon doesn’t burn. Check to see that internal temperature is about 145-150 degrees F before removing from the oven. This means the pork will be juicy and slightly pink in the middle.
Rest: Transfer the pork to a cutting board and let rest 10 minutes before slicing. Brush with sauce once more before slicing into thick slices. Serve with remaining Chili Dijon Sauce on the side.