Preheat the oven to 300°F.
Add the sugars, onion powder, kosher salt, paprika, black pepper, ground mustard, cumin, cayenne pepper, and nutmeg to a small mixing bowl and whisk to combine.
You will need to carefully remove the opaque membrane that covers the underside of the ribs with a small knife. Lift the corner edge of the membrane and carefully slide the knife's edge underneath. Continue lifting the membrane up while cutting. Once you have this process started, you should be able to use your fingers to pull the membrane away.
Tear a long piece of heavy-duty aluminum foil, long enough to cover the rack of ribs. Lay it over a baking sheet.
Place the rack of ribs, meaty side down and bone side up, in the middle of the foil.
Generously spread ⅓ of the dry rub over the bones and rub into the bones. Flip the ribs over and spread the remaining dry rub over the top of the ribs and rub.
Crimp the seams of the foil together. Roll up the ends of the foil to seal.
Bake for 2 hours 30 minutes.
Remove the ribs from the oven. Increase the oven temperature to broil.
Add ½ cup barbecue sauce and the 3 tablespoons of brown sugar to a small mixing bowl. Whisk to combine.
Carefully open the foil. Spoon and spread the sauce over the top of the ribs.
Return the ribs to the oven to broil the sauced ribs for 2 to 3 minutes, just to caramelize the sauce. Remove the ribs from the oven and allow them to rest for 10 minutes before separating the individual ribs.