Step 1

Preheat the oven to 325°F (163° C).

Step 2

To remove the membrane from the back of the ribs, slip the tip of a small, dull knife, such as a table knife, in between the membrane and bone anywhere along the edge of the slab of ribs. Lift the knife to separate the membrane from the bone, then grab the membrane with one hand through a paper towel (the towel helps keep the membrane from slipping out of your grip) and rip it off.

Step 3

Spread the onion slices evenly on a rimmed baking sheet. Place the ribs, bone side down, on the onion. Pour the beer over the ribs, season with salt and pepper, and cover tightly with foil. Bake, undisturbed, for 2 hours.

Step 4

Prepare a fire in a charcoal grill and let the coals burn to a gray ash with a faint red glow, or until you can hold your hand 3 to 4 inches above the fire for no more than 6 seconds.

Step 5

Brush both sides of the baby back ribs with the barbecue sauce and place the ribs, meat side down, over the coals. Grill the ribs, basting the top-facing side several times, until slightly charred, 10 to 15 minutes. Turn the slab of ribs and liberally baste the cooked side. Treat the slab gingerly, so as not to lose any of the luscious meat to the flames of the grill. Close the lid of the grill and cook the ribs, basting often, 10 to 15 minutes longer. Cut the slabs into individual ribs, pile them onto a large platter, and serve warm.