In a blender or food processor, combine the mayonnaise, sour cream, ranch mix, milk, fresh cilantro, minced garlic, diced jalapeños, fresh lime juice, and ground cumin. Blend until smooth and creamy, about 1-2 minutes. If the dressing is too thick, add a splash more milk to reach your desired consistency. Taste and adjust seasonings as needed. Transfer to a bowl and set aside. I like to make the dressing first so the flavors have time to meld together while I prepare the salad components.
In a large bowl, combine the chopped romaine lettuce, diced orange bell pepper, rinsed black beans, halved cherry tomatoes, grilled corn, and cubed avocados. Gently toss everything together to distribute the ingredients evenly. I find that adding the avocados last and tossing gently prevents them from breaking apart and getting mushy. Add the grilled chicken slices on top of the salad mixture.
Pour the cilantro-ranch dressing from Step 1 over the assembled salad from Step 2, adding as much or as little as you prefer—I typically use about three-quarters of the dressing and reserve the rest on the side so guests can add more if desired. Sprinkle the sliced green onions and chopped fresh cilantro over the top, and finish with the lime zest for brightness and aroma. Toss gently just before serving to coat everything evenly with dressing.