To make the slather, combine all the ingredients in a small bowl and stir to blend well.
Prepare an indirect fire in your grill, with a hot fire on one side and no fire on the other. For a charcoal grill, soak 1 cup wood chips in water for at least 30 minutes and place directly on the hot coals when ready to plank. For a gas grill, place 1/2 cup dry wood chips in a smoker box (or wrap chips in aluminum foil and poke holes in the foil) and place over direct heat in the back of the grill.
Sear the steaks over the hot fire for 3 or 4 minutes on one side, and then place the steaks on the plank, uncooked side down. Top each fillet with about 1 tablespoon of the slather.
Place the plank on the indirect side of the grill. Close the lid and cook until a meat thermometer inserted into the thickest part of the steak registers 130°F for medium-rare, about 20 minutes.
Oven-Smoke Planking: Preheat the oven to 400°F.
Place 1/2 cup dry wood chips in a small metal pan on the bottom rack of the oven and let smolder for about 15 minutes before planking. Meanwhile, in a medium-hot skillet, sear the tenderloin on all sides. When the smoke begins to fill the oven, place the seared steaks on the plank in the middle of the oven. Oven-plank for 20 minutes, or until a meat thermometer inserted into the thickest part of the steak registers about 130°F for medium-rare.
