Line a rimmed baking sheet with nonstick foil or parchment paper for easy cleanup. Preheat the oven to 275°F.
Place the slab of ribs in the pan, bone side down. Spray it with oil and sprinkle with salt and pepper. Bake the ribs for one hour, uncovered.
Meanwhile, in a medium bowl, mix the barbecue sauce ingredients: tomato paste, balsamic vinegar, smoked salt, black pepper, garlic powder, smoked paprika, and maple syrup. Divide the sauce into three small bowls and set aside.
After one hour, remove the ribs from the oven and carefully drain the pan juices. Brush the top of the ribs with ⅓ of the BBQ sauce and return them to the oven. Wash the basting brush with warm, soapy water.
Repeat this after one more hour: Brush the top of the ribs with another ⅓ of the BBQ sauce and return them to the oven. Wash the basting brush with warm, soapy water.
After one more hour, brush the ribs with the remainder of the sauce. Reduce the oven to 250°F and return the ribs to the oven for one final hour.
After 4 hours in the oven, the meat will be super tender and fall off the bones. The top will be dark and deliciously caramelized. Remove the slab from the oven, cut it into individual ribs, and serve.
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Notes
The BBQ sauce darkens and caramelizes in the oven. It should be very dark but not burned. If you think it's darkening too much, loosely cover the ribs with foil during the last one or two hours of baking.
It's easiest to cut the ribs when you place them on a cutting board bone-side up and use a sharp chef's knife. When they're bone-side up, it's easy to locate the bones and cut in between them. You can cut the slab into one or two rib portions.
Pork spare ribs have a thin membrane on the back of the slab. Whether you remove it is a personal choice. I don't. It becomes nicely charred and caramelized after baking, and I enjoy pulling it off the ribs and eating it. However, some people don't like its texture and prefer to remove it. This is easier to do if you pull on its edge with a paper towel.
You can keep the leftovers in an airtight container in the fridge for 3-4 days. Gently reheat them, covered, in the microwave at 50% power or uncovered in a 350°F oven. You can also freeze the cooled leftovers for up to three months.