Sauté onion in oil over medium heat for 4 minutes or until tender. Stir in black beans and next 8 ingredients. Bring to a boil and reduce heat. Simmer for 15 minutes. Combine baking mix, egg, and milk in a medium-size bowl; stir until smooth. Fold in ½ cup cheese and ¼ cup cilantro. Pour over chicken mixture. Bake at 400 degrees for 20 minutes or until a knife inserted comes out clean. Sprinkle with remaining cheese and cilantro. Sprinkle avocado slices with lime juice and arrange on top. Garnish with sour cream.