1. Immerse each avocado half fully in fresh lemon juice to avoid browning (you can skip this step if preparing this dish right before serving).
2. Combine the mashed avocado with one teaspoon lemon juice, mayonnaise, and salt to taste, and set aside.
3. Discard the centers of the plum tomatoes and dice very finely.
4. Dice the shallot very finely and combine with the tomatoes in a bowl.
5. Add the apple cider vinegar, olive oil, and some kosher salt to taste. Mix well to coat.