Cook 1 package Knorr Mexican or Spanish rice according to the package directions; keep warm.
Make the avocado ranch sauce: in a blender combine ½ cup sour cream, ⅓ cup buttermilk, ½ cup ripe avocado, 1 teaspoon chopped garlic, 2 teaspoons lime juice, and 2 teaspoons dry ranch dressing mix. Puree until smooth. Transfer the sauce to a small airtight container and refrigerate until assembly.
Prepare the tomato for guacamole and assembly: chop 1 tomato. Measure out 1 teaspoon of the chopped tomato and set it aside for the guacamole; reserve the remaining chopped tomato for the bowls.
Make the guacamole: in a bowl mash 2 small or 1 large avocado. Add 2 teaspoons lime juice, 1 tablespoon diced red onion, 1 teaspoon diced tomato (the teaspoon you set aside), and ½ teaspoon salt. Stir gently to combine and set aside.
Season the chicken: pat ½ pound chicken breast dry and season both sides with ½ teaspoon salt and ¼ teaspoon ground black pepper.
Cook the chicken: heat a skillet over medium heat and add 2 teaspoons olive oil. When the oil is hot, add the seasoned chicken and cook about 7 to 9 minutes per side, or until cooked through (time will vary by thickness). Remove the chicken from the skillet and let rest for about 3 minutes, then slice into thin strips.
Drain and rinse 1 15-ounce can black beans and set aside.
Prepare remaining assembly components: measure ½ cup shredded Cheddar cheese, ½ cup low fat sour cream, and 2 cups shredded lettuce.
Assemble the bowls: divide the cooked rice among bowls. Over the rice add sliced chicken, drained black beans, guacamole, the remaining chopped tomato, ½ cup low fat sour cream (divided as desired), ½ cup shredded Cheddar cheese, and 2 cups shredded lettuce.
Finish and serve: drizzle the avocado ranch sauce over each bowl to taste and serve immediately.