Wash and then very thoroughly dry the lettuce (you don't want it wet at all). Chop it into very small pieces -- you'll want smaller pieces of lettuce than a typical salad. Add the chopped lettuce to a large bowl.
Add in the drained tuna, drained and rinsed black beans, drained corn, diced avocados, diced red bell pepper, diced red onion, and coarsely chopped cilantro. Toss to combine.
For the Dressing
Zest and juice the lime. Add the juice (you should have about 2-3 tablespoons juice) and zest (about 1/2 teaspoon zest) to the blender along with the prepared ranch seasoning mix, minced garlic, chopped tomatillo, and cilantro (Just cut off the large stems, but the rest is fine).
Add 1/2 a jalapeno* (a full one for more kick), the mayo, and the buttermilk. Add some salt and pepper to taste (I use 1/4 to 1/2 teaspoon of salt and 1/8 teaspoon pepper).
Blend until completely smooth. Taste and adjust to personal preference (more lime, more cilantro, more garlic, salt/pepper)
Dress the salad right before enjoying (do not dress in advance; do not dress anything you want to have leftover), toss the salad together with the dressing.
Season with salt and pepper if needed and then add in any fresh cilantro or lime juice to taste. Enjoy immediately after being dressed.
Notes
* Reduce heat by eliminating the seeds; keep the seeds in for more heat.