1Mix brown sugar, paprika, garlic powder, onion powder, black pepper, and salt in a small bowl to make your dry rub.
2Rub the spice mixture all over the ribs, coating both sides generously. Let sit for 15 minutes.
3Add water, halved onion, and garlic cloves to your instant pot. Place the trivet inside.
4Cut the rib rack to fit and layer on the trivet. Set to high pressure for 25 minutes.
5While ribs cook, make the bbq sauce: whisk together tomato paste, apple cider vinegar, worcestershire, ketchup, molasses, mustard powder, and cayenne in a saucepan over medium heat.
6Simmer the sauce for 5 minutes, stirring occasionally, until it thickens slightly. Season to taste with salt and pepper. Set aside to cool slightly.
7Quick release the instant pot. Carefully remove ribs and pat them dry with paper towels.
8Line a baking sheet with foil. Arrange ribs meat-side up and brush generously with bbq sauce.
9Broil on high for 4-5 minutes, watching carefully, until the sauce caramelizes and edges char slightly.
10Brush with another coat of sauce, let cool for 3 minutes, then serve with extra sauce on the side.