Step 1

Preheat oven to 300°. Pat ribs dry; season generously on both sides with salt and pepper. Wrap each rack in a double layer of foil, crimping edges tightly, and place on a rimmed baking sheet. Bake until meat yields easily when pierced with a skewer but is not quite falling off the bone, 2–2½ hours. Remove from oven and let sit inside foil until cool enough to handle.

Step 2

Meanwhile, pulse scallions, garlic, and ginger in a food processor until finely chopped. Heat 2 Tbsp. oil in a medium saucepan over medium. Add aromatics and cook, stirring often, until softened, golden, and starting to stick to bottom of pan, about 4 minutes. Add gochujang, ketchup, lime juice, plum sauce, sugar, fish sauce, soy sauce, and ¼ cup water and stir until smooth. Bring to a simmer and cook, stirring often to prevent scorching, until thick, 5–10 minutes. Remove sauce from heat; season with salt.

Step 3

Prepare a grill for medium-high heat; lightly oil grates. Remove ribs from foil (if desired, stir cooking liquid into sauce) and cut racks in half crosswise. Place ribs on grill, meaty side down, and grill until lightly charred, about 1 minute. Brush ribs with sauce and turn over. Brush with more sauce and continue to grill, turning ribs one or two more times, until sauce is lightly charred in spots, about 2 minutes. Transfer to a cutting board and let rest 5 minutes before slicing into individual ribs.

Step 4

Serve ribs with remaining sauce and lime wedges.