In saucepan of boiling water, bring chicken, onion, garlic, coriander and salt to boil. Reduce heat to medium-low; cover and cook until chicken is no longer pink inside, about 20 minutes. Drain, discarding onion, garlic and coriander. Let cool on cutting board for 10 minutes. Using fingers or 2 forks, shred chicken. Set aside.
Meanwhile, in separate saucepan, heat oil over medium-low heat; fry onion and garlic until onion is translucent, about 5 minutes. In blender, purée tomatoes, chipotle peppers and tomato paste; add to onion mixture. Simmer over low heat, stirring occasionally, until thickened and reduced, about 35 minutes.
Add reserved chicken, oregano and cook until thickened, about 10 minutes. Remove from heat; stir in coriander. (Make-ahead: Cover and refrigerate for up to 24 hours.).
Corn Spoon Bread:.
Line 8-inch (2 L) square metal cake pan with parchment paper; set aside.
Meanwhile, in large bowl, whisk together corn flour, baking powder and salt. In separate bowl, whisk together egg, sour cream and oil; stir in corn and cheese. Stir into flour mixture until combined. Scrape into prepared pan. Bake in centre of 400°F (200°C) oven until golden and cake tester inserted in centre comes out clean, about 25 minutes. Let cool slightly in pan on rack. Cut into 12 triangles.
Serve chicken mixture over corn bread. Garnish with avocado, sour cream and coriander leaves. Makes 6 servings.