Begin by toasting the fennel seeds in a dry pan over medium-high heat. Toast for 4 or 5 minutes, until they become fragrant; toss occasionally as they toast so they don't burn. Transfer to a spice grinder and grind to a fine powder.
Rinse the ribs in cool water and pat dry with paper towels. Unless your butcher has already done so, remove the membrane from the back and trim off any excess fat. Combine the dry rub ingredients in a small bowl and mix well. Coat both sides of the meat with a heavy layer of the dry rub. Refrigerate overnight, uncovered.
Heat half the grill to medium to create indirect heat.
Set out a few large sheets of foil (one for each slab). Divide the sliced onion and thyme between the sheets and spread them out evenly. Place 1 rack on each sheet, meat side down, and fold the foil over to form a sealed pouch around each rack. Place each pouch on a roasting tray (to keep foil from getting pierced and letting steam out) and grill it over indirect heat for 2 hours.
Open the foil and peel it back so the ribs are exposed. Carefully turn the meat over so the meat side is on top, then cook for 1 1/2 hours with the foil open. When done, the ribs will be tender and the meat will have shrunk back from the bones.
For the last 10 minutes, remove the ribs from the foil and place them directly on the hot side of the grill. Brush with the Orange BBQ Sauce and allow the heat to caramelize the sauce and crisp the exterior, then flip the ribs and brush the other side for the final 5 minutes. Serve with additional sauce.
Orange BBQ Sauce:
Yield:about 1 1/2 cups
Whisk together all the ingredients in a medium saucepan and simmer over medium-low heat for 25 to 30 minutes, until it reaches a ketchup-like consistency.
Prep-Ahead Tip: Make the dry rub ahead of time and keep in an airtight container. Dry-rub the ribs the night before and store in the fridge. The BBQ sauce can be made 1 or 2 days in advance and stored in an airtight container in the fridge.
Copyright 2014 by Guy Fieri, by arrangement with HarperCollins Publishers