To hard-boil eggs: PLACE eggs in saucepan large enough to hold in a single layer. ADD cold water to cover the eggs by 1 inch. HEAT over high heat just to boiling. REMOVE from burner and COVER pan. Let eggs STAND in hot pan for about 15 minutes for large eggs (12 minutes for medium eggs; 18 minutes for extra-large). DRAIN water from pan.
SHOCK the eggs by setting in a bowl of ice water to cool them immediately. PEEL eggs once they are completely cooled.
Roughly CHOP eggs and COMBINE with cilantro, celery, Dijon mustard, Sriracha, pepper and mayonnaise and MIX well. SET aside.
TOAST bread slices. MOUND egg mixture onto toast. PLACE sliced tomatoes and onion slices on top of egg mixture and TOP with avocado slices. GARNISH with cilantro. SERVE immediately.
Mash the avocados and use as a spread on the toast before adding the egg mixture.