Preheat oven to 300*F
Pat rib rack dry with paper towels and generously sprinkle with Al Pastor Rub, being sure to cover completely.
Double wrap ribs tightly in tin foil and place them on a baking sheet. Bake for about 2 hours until just fork tender but not quite separating from the bone yet.
Remove ribs from the oven, unwrap and chill in the fridge for at least 2 hours or overnight.
Make the sauce: In a medium-sized pan, heat butter or oil over medium-high heat. Add onions and garlic, sauté for about 2 minutes until they start to take color. Add the rest of the ingredients to the pan and whisk together to combine. Bring to a simmer, then reduce the heat to medium-low. Simmer for about 15- 20 minutes until it starts to thicken. Carefully pour the sauce into a blender and puree. Set aside.
Heat charcoal or gas grill to medium-high heat.
Cook ribs on the grill over direct heat flipping frequently and brushing each side with the bbq sauce until both sides are well charred, about 10 minutes.
Remove from the grill and let the ribs rest for 10 minutes. Slice ribs between each bone and serve immediately with extra bbq sauce and your favorite sides.