Place the rack of ribs on a sheet pan. Season generously with salt and pepper and place in the fridge, uncovered, for about 1 hour or up to overnight before you want to cook.
After an hour (or however long you left the rack in the fridge), prepare a charcoal or gas grill for high heat, making sure to clean the grates and lightly slick them with a kitchen rag or brush dipped in canola oil. Cut the rack into individual pieces and return to the sheet pan. Drizzle 2 tablespoons of canola oil over ribs, rub the oil all over and set ribs aside.
In a bowl or another sheet pan, season the scallions and serrano chile with salt, drizzle the remaining 1 tablespoon oil over and toss. Place scallions and serrano chile on a hot grill and cook, turning occasionally, until softened and deeply charred, about 3 to 5 minutes. Transfer to a cutting board. Cut the scallions into ½-inch pieces, chop the serrano pepper finely and place in a bowl. Add the olive oil, parsley, vinegar, capers, sugar and garlic. Season with salt and pepper, toss well to combine and set aside.
Place the ribs on the grill and cook, turning a few times, until golden brown with some light charring and cooked through, about 15 minutes.
Transfer ribs to a serving platter and spoon the charred scallion sauce over them.