In a medium bowl, whisk buttermilk, mayonnaise, minced chives, garlic, lime juice and zest together. Stir in the crumbled cotija. Season with kosher salt and fresh ground pepper. Store in fridge until ready to serve.
Heat grill to medium-high. Brush corn, green onions and jalapeño with avocado oil and season with kosher salt and fresh ground pepper.
Arrange on the grill and cook, turning occasionally, until corn, green onions, and jalapeños are grilled to your liking. Approximately 8 to 10 minutes.
Transfer corn, green onions, and jalapeño to a cutting board and let cool slightly. Remove corn from cob, chop green onions and jalapeño.
In a large bowl or serving platter, arrange lettuce with cut corn kernels, green onion, and jalapeño. Add avocado slices and tomatoes. Serve with buttermilk dressing.