Broth - Heat a large soup pot over medium-low heat. Place the 2 halved onions and garlic heads cut-side down, cover pot and cook, untouched, until bottoms are dark brown. About 10 minutes. Meanwhile, grind the peppercorns, cloves, cinnamon and oregano in a spice mill or mortar and pestle. When onion/garlic is done, add chicken broth and bring to a boil. Reduce heat to a steady simmer and add spices. Cut long strips of peel from 2 limes and the grapefruit. Reserve juices from 1 lime and grapefruit. Simmer pot partially covered for 2 hours. Strain the broth and measure 2 quarts. Top with water if necessary. Return to pot.
Tortilla Strips - Leave tortillas out to dry for about 30 minutes. Cut dried tortillas in half and then slice stacked halves crosswise into ¼-inch strips. Over medium-high, put ⅓ cup oil in a skillet. When hot, add tortilla strips and fry, turning often, until they become crispy. Drain on paper towels.
Prepare Soup - Pour oil out of pan, leaving a thin coating. Over medium-heat, cook chopped onion and pepper, stirring until soft and just beginning to brown. About 5 minutes. Add reserved lime and grapefruit juices and scrape everything into broth along along with the chicken breast. Cook 30 minutes until chicken is cooked through. Remove chicken and let cool slightly before removing the skin and bones. Break up meat into large shreds. Put aside. Salt broth to taste, about 2 tsp.
Serve - Slice remaining whole lime into 8 wedges and 1 of the peeled limes into ¼-inch disks. Divide chicken shreds, tortilla strips, and lime slices between serving bowls. Ladle hot broth over everything, sprinkle coriander on top and serve with lime wedges on the side.