For the avocado ranch dressing: Add the avocado, buttermilk, mayonnaise, lemon juice, chives, dill and parsley to the jar of a blender and puree until smooth and bright green. Set aside.
For the salad: Heat a grill or grill pan over medium-high heat. Drizzle each ear of corn, peppers and green beans with olive oil, then sprinkle with a pinch of salt and a few grinds of black pepper. Add the vegetables to the grill in a single layer. Grill on all sides until char marks begin to appear, a few minutes. Remove vegetables from the grill and set aside. When the corn is cool enough to handle, slice kernels off the cob.
Put the romaine into a large salad bowl along with the cherry tomatoes, onions, green beans, corn, peppers, 1 teaspoon salt and a generous amount of black pepper. Drizzle all of the dressing over the top and toss until well combined. Sprinkle with more chives, dill and parsley before serving, if desired.