Make the rub, whisk together all the pimentón, garlic powder, salt, onion powder, ancho chilli powder, pepper, oregano and thyme.
Place the ribs in a shallow pan and generously rub the spice mix all over the racks of ribs, on both sides. Cover and refrigerate for 1 hour.
While the ribs are marinating, make the sauce: in a blender, add the tomato sauce, chipotle chillis including some of the sauce, the onion quarters and the garlic. Blend until smooth and set aside.
Heat the olive oil in a saucepan over medium heat. Once the oil is hot, add the minced onion and stir until translucent and fragrant, about 5 minutes. Carefully add the blended tomato sauce. Bring the sauce to a boil over medium heat and cook until it has reduced by a third, about 10 minutes. Add the vinegar, Worcestershire sauce, brown sugar, Tabasco, tomato paste, and salt and stir to combine. Lower the heat and continue to cook for another 20 minutes, until the sauce has thickened. Remove from the heat and allow to cool.
Preheat your grill to 135°C if using a pellet grill. If you don’t have a pellet smoker, you can use your oven for this step.
On a work surface, spread out 2 large sheets of aluminium foil on top of each other (big enough to wrap each of the racks individually). Working one at a time, remove a rack of ribs from the fridge and place it on top of a piece of foil. Pour about half of the cooled sauce over the top and wrap the ribs in the foil, pleating the edges so the package seals well. Repeat with the second rack.
Place the ribs in the middle of your preheated grill or oven, meat side up, and cook for 3 hours. After 3 hours, carefully uncover the ribs. Reserve 240ml of sauce in a serving bowl for dipping later. Liberally brush the ribs with the rest of the sauce. Cover the ribs again and allow to cook for 1 more hour.
Remove the ribs and prepare your grill for hot direct cooking. Make sure the grates are clean. If using a pellet grill, raise to high heat and crisp your ribs for about 4 minutes on each side, until the ribs are fall-apart tender and really saucy. If using a pellet grill, remove the ribs, carefully remove the grill grate using two tongs and grilling gloves, and add more charcoal until you reach a higher temperature, and place the ribs back on the grill to crisp up then.
Once the ribs are crispy, using either two spatulas or a spatula and tongs, remove the ribs to a platter and serve right away, with the leftover sauce on the side. The ribs may be so tender, that they will shred when you try to serve yourself.