Make the turkey loin marinade: Whisk together the olive oil, soy sauce, lemon juice, Dijon mustard, brown sugar, onion powder, paprika, thyme, sage, black pepper, and garlic in a measuring cup. Reserve 3 tablespoons of the marinade and set it aside.
Place the tenderloins into a sealable plastic bag or a bowl that will allow them to be fully submerged.
Pour the marinade over the tenderloins and seal the bag or cover the bowl, trying to remove as much air as possible. Marinate for at least an hour or for up to 8 hours in the refrigerator.
Remove the turkey tenderloins from the refrigerator 30 minutes before grilling.
To start grilling, preheat the grill to 400 degrees (or medium-high heat.) Scrape the grates of any possible food residue, then generously coat the grates with a wad of paper towel that has been dipped in olive oil.
Remove the tenderloins from the marinade and make sure to shake off any excess marinade.
Place the turkey tenderloins on the grate and grill, covered, for 8 minutes. Flip and cook for an additional 6-7 minutes with the grill covered or until the internal temperature reaches 155 degrees F. in the thickest part of the tenderloin. (There will be carry-over cooking when removed from the grill so that the internal temperature will reach 165 degrees F.) If the tenderloin appears to be burning, move it to the cooler side of the grill, cover the grill, and continue to cook until done.
Remove from the grill, and tent with aluminum foil, and let it rest for 10 minutes.
Slice your grilled turkey tenderloin to your desired thickness, brush it or drizzle with the remaining marinade, and serve.