See note #1 on removing membrane from baby back ribs. This step is important! If you do not remove the membrane you will end up with tough chewy ribs.
In a bowl, add all of the spices. Mix with a spoon until combined. Use the back of the spoon to press out any large clumps.
Tear two sheets of aluminum foil, long enough to pass the sides of each rack of ribs. Place one rack, curved side down (the side that you removed the membrane will be facing down), per foil. Using a brush, spread 1 tbsp of yellow mustard on each rack. Sprinkle a 1/4 of the dry rub on the rack and using your fingers, spread. Flip the racks over and repeat mustard and dry rubbing.
Once the ribs have been rubbed, carefully wrap the aluminum foil closed. I add an extra sheet of foil around the center, to ensure the ribs are fully sealed. [See note #3]. Place them curved side down, so that the side you removed the membrane, is facing down.
Place in the refrigerator and allow to marinate for 2 hours.
Preheat oven to 275℉. Remove ribs from refrigerator, place on baking sheet curved side down(the side you removed the membrane, is facing down) and bake for 2 1/2 hours. If you see liquid start coming out onto the baking pan, carefully wrap with more foil.
Preheat grill to 350℉. After the ribs have baked low and slow in the oven for 2 1/2 hours, remove from the oven. Carefully open the foil. Using a meat thermometer, take the temperature in between the bones. Pork is technically done at 145℉, but if you want fall-off-the-bone ribs the temperature shoud be at 190°F-200°F.
Using your favorite BBQ sauce, brush the top of the ribs with the sauce. Since the ribs will be so tender, they may fall apart transferring to the grill. Use two sets of tongs (one on each side) to help them stay together when flipping. Place the ribs sauced side down, onto the grill. While that side chars, brush BBQ sauce on the side facing up. Grill for about 5 minutes per side, until ribs get charred grill marks. Remove from grill.
To slice, place the ribs curved side up, so you can see the bones. Slice in between bones and serve with additional BBQ sauce.