In a small bowl, add the mayonnaise, buttermilk, 1 teaspoon of the salt, the agave, cumin, lime juice, zest and 4 to 5 turns pepper.
Whisk to combine and set aside. In a medium bowl, add the chayote and the remaining 1 teaspoon salt, then toss to mix and place on a paper-towel-lined baking sheet for 10 minutes.
In a large bowl, add the drained chayote, carrots, onions, bell peppers and scallions. Pour the dressing over the vegetables and mix to combine. Serve immediately or cover and refrigerate until ready to use.