Preheat the oven to 425 degrees F. Season the hens with salt and pepper and place skin-side up in a roasting pan. Roast until the skin is slightly crisp, about 25 minutes.
Meanwhile, combine the soy sauce, hoisin sauce, vinegar, ginger, 1 chile pepper and 1/4 cup water in a pot. Cut half of the scallions into large pieces, and zest and juice half of the grapefruit; add to the pot. Simmer, stirring occasionally, until thick and syrupy, about 20 minutes. Brush the hens with a few tablespoons of the sauce and continue roasting until golden, about 8 more minutes.
Peel and segment the remaining grapefruit half and place in a bowl. Thinly slice the remaining scallions and mince the remaining chile pepper; add to the bowl along with the bok choy, sesame oil, and salt and pepper to taste. Place half a hen on each plate. Stir any pan drippings into the remaining sauce and drizzle over the top. Serve with the bok choy salad and garnish with sesame seeds.
Photograph by Antonis Achilleos