For the dressing
In a small bowl or jar, combine the vinegar, olive oil, mustard, and salt. Whisk until emulsified and set aside. (You can also make this in a blender if you prefer — simply combine and blend.)
For the salad
Bunch or stack a few kale leaves at a time, then chop them into small pieces. You don’t have to get too particular here, but the smaller the cut, the easier and more pleasant the salad is to eat.
In a large bowl, add the kale with a pinch of salt and ¼ of the avocado. With clean hands, massage the avocado into the kale by rubbing it between your hands until the avocado is thoroughly worked into the kale, and the kale has no dry spots remaining. Let the kale sit while you prepare the remaining ingredients: slice the tomato and remaining avocado; chop the cucumber; and dice the red onion. Stack a few large leaves of basil, roll them into a cylinder, then slice thinly. Toss all ingredients together, then drizzle with dressing to taste. Serve.