Add the oil, brown sugar, soy sauce, mustard, lemon juice, garlic, and salt to a medium bowl. Whisk until smooth and the sugar has dissolved.
Set aside 2 tablespoons of the marinade.
You will use the reserved marinade to drizzle over the cooked pork at the end.
Pour the remaining marinade along with 2 tablespoons of water into a 1-gallon zip-top bag or baking dish.
Using a fork, lightly poke holes all over the pork tenderloin, then add to the bag or dish, press as much air out of the bag (if using) as possible, or cover with plastic wrap (if using a baking dish).
Refrigerate for at least 30 minutes, or up to 12 hours, flipping occasionally.
Grilling Instructions
Heat a grill to medium-high. Allow to preheat for 15 minutes. Remove the pork tenderloin from the marinade, shake off any excess, and discard any remaining marinade from the bag or dish.
Grill, with the cover down, until the internal temperature registers 145F (medium rare) or 160F (medium), about 5-7 minutes per side.
Remove to a cutting board and allow to rest for 5-10 minutes. Slice against the grain into 1 inch slices, or dice into pieces (if using for taco), serve, drizzled with reserved 2 tablespoons marinade, and serve.
Baking Instructions
Preheat your oven to 400°F. Add the pork tenderloin along with marinade to a medium baking dish. If you have a large baking dish or rimmed sheet pan, omit the marinade as it will spread and possibly burn.
Bake for 25-35 minutes, until the internal temperature registers 145F (medium rare) or 160F (medium).
Remove to a cutting board and allow to rest for 5 minutes. Slice against the grain into 1 inch slices, or dice into pieces (if using for taco), serve, drizzled with reserved marinade, and serve.
Notes
Check to make sure your pork is done with a meat thermometer for an internal temperature of 145° F for medium rare and 155° F for medium with a 3 minute rest time.