Mash avocados the clean and easy way. place halves in a heavy-duty zip-top plastic bag; seal and squeeze.
Spread 1 side of each toast slice evenly with mayonnaise; top 4 toast slices evenly with onion slices and next 5 ingredients.
Spoon mashed avocado on top. Sprinkle with salt and pepper. Top with remaining toast slices, and serve immediately.