Preheat the oven to 350 degrees F.
Mix together the brown sugar, mustard, paprika, smoked paprika, black pepper and garlic salt together in a small bowl. Be sure to break up any lumps with your fingers. Reserve 1 tablespoon of rub in a small bowl for serving.
Remove the silver skin from the underside of the ribs by sliding your fingers under the thin membrane and pulling it off. Repeat with the second rack. Rub the ribs with the seasoning on both sides. Place in a single layer in a large roasting pan and cover tightly with heavy-duty foil. Place the roasting pan into the oven and bake until the ribs are tender, about 1 hour 15 minutes.
Preheat the grill to medium heat.
Grill the ribs on each side for 15 minutes, watching and flipping when necessary, for 30 minutes total. Baste with Jamie's BBQ Sauce the last 10 minutes. (The sauce goes on at the end because it is sugar based and you don't want to burn your ribs.)
Let the racks rest 5 minutes before slicing into individual ribs. Sprinkle the cut ribs with the reserved dry rub and serve along with extra sauce.
Heat the oil in a medium saucepan over medium-high heat. Add the onions and stir until softened. Add the vinegar, ketchup, brown sugar, mustard, Worcestershire sauce, chili powder, cayenne pepper and some salt and black pepper to the saucepan and simmer on medium-low heat until thickened, about 35 minutes.