In a small saucepan, heat apple cider vinegar and erythritol over high heat for 4 minutes until boiling.
Add onion and remove saucepan from heat.
Let set while preparing remaining ingredients.
Drain when ready to use onions.
Combine cream cheese and capers in a small bowl.
Cover and refrigerate until ready to use.
Preheat air fryer at 180°C for 3 minutes.
Place avocado halves, cut sides up, in ungreased air fryer basket and cook for 4 minutes.
Transfer avocados to two medium plates and garnish with cream cheese mixture, smoked salmon, pickled onions and tomato halves.
Serve.