Cut off the silver fat on each pork loin. It’s okay to leave some, but cut off visible large pieces.
Use a sharp knife to Butterfly the pork loins. Season each side with salt and pepper—don’t over salt the meat because the marinade will be seasoned well with soy sauce.
Add the mustards, honey, soy sauce, and harissa to a bowl and mix well. Taste for additional honey, soy sauce, harissa—add for more sweetness, salt, and spice.
Add marinade and tenderloins to a zip top bag and toss to coat. Allow to sit overnight or up to 3 days.
Preheat the grill to 500-F to sear, and then knock the heat down to cook.
Grill the tenderloins until they reach an internal temperature of 145-F, then let them rest for 10 minutes before cutting and serving.