Soak cedar plank in water for an hour or so. Remove skin from chicken thighs. Sprinkle both sides of chicken pieces with onion powder, granulated garlic, paprika, salt and pepper.
On a gas grill, place soaked cedar plank directly over medium heat and allow wood to get hot, 3-5 minutes.
Place chicken directly on the cedar plank and cook over medium-low heat (this will depend on the grill, so you may have to adjust the temp to be sure the plank stays hot enough).
As the chicken cooks, rotate one or two pieces at a time and cook directly on the grill for a few minutes, then back to the plank. This will help the chicken cook through.
During the cooking process, the wood plank will char and smoke which is ideal. If the wood should begin to catch fire, simply spray those parts with a little water.
Once the chicken is cooked through, transfer to a rimmed baking sheet and allow to cool until easy to handle. This is the time to make the sauce.
For the Sauce:
Process onion and water in food processor until pureed. Pass mixture through fine-mesh strainer into liquid measuring cup, pressing on solids; this should produce approximately 3/4 cup strained onion juice. Discard solids.
Whisk onion puree, ketchup, apple cider, sugar, Worcestershire, mustard, molasses, pepper, cider vinegar, and liquid smoke together in medium bowl.
Heat oil in nonreactive saucepan over medium heat until shimmering; add garlic, chili powder, and cayenne and cook until fragrant, about 30 seconds.
Stir in ketchup mixture; increase heat to medium-high, bring to boil, reduce heat to medium-low, and simmer, uncovered, until flavors meld and sauce is slightly thickened, about 15 minutes. This will yield approximately 4 cups of sauce.
Remove chicken from the bone with fingers. Remove any gristle, fat, or connective tissue so that only the meat remains. Pull chicken meat apart so that it is in small strips and pieces.
To serve, toss chicken with desired amount of sauce in a separate large skillet or oven dish--add as little or as much sauce as you prefer.
Extra sauce can be stored in the refrigerator. Heat chicken with sauce in the skillet over medium-low heat or in the oven at 350° until incorporated and heated through. Serve on Sprouted Wheat Buns with coleslaw on top.