Combine water, vinegar and sugar in microwave safe bowl. Microwave on HIGH 2 minutes or until boiling.
Add celery and let stand 15 minutes. Drain.
Combine avocado, mayo, juice, mustard, salt and pepper in bowl. Stir well until smooth.
Peel eggs and slice in half lengthwise. Chop the eggs.
Gently stir eggs, celery and sunflower seed into avocado mixture.
Top 4 slices of bread with about ½ c. of egg mixture, ¼ c. arugula, 1 tomato slice and remaining bread slices.
Notes
****To prevent avocado from browning in leftover egg salad, place any remaining salad in a bowl and cover surface with plastic wrap. Then cover entire bowl tightly with plastic wrap.
Recipe by Geaux Ask Alice! at https://geauxaskalice.com/amazing-avocado-egg-salad-sandwiches/