In a large saucepan over medium heat, sautee onion and garlic in oil until translucent, about 4-5 minutes.
Stir in chicken and brown, then add stock, lime juice, Ro-Tel (with juice), salt and pepper. Gently simmer until chicken is cooked, 3-5 minutes. Stir in avocado and cilantro and heat through, and adjust seasonings to taste.
(If you wish to thicken the soup a bit, use an immersion blender or mash some of the avocado by hand. You can also add potato flakes if you want a thicker broth.).
Ladle soup into bowls and garnish with a dollop of sour cream and cheddar cheese, a sprinkle of additional cilantro, with lime wedge and tortilla chips on the side. Enjoy!