Prepare the pickled onions. Slice onions very, very thinly. Bring vinegar, water, salt, and sugar to boil. Place onions in mixture and stir and let come to room temperature. Refrigerate for at least 30 minutes (an hour is even better).
Mash the avocado in a bowl and add a little lemon juice and salt.
Toast the bread. If you're camping and your grill is hot, throw it on the grill for a minute or so on each side.
Spread the avocado mixture on the toasted bread and layer on a plate. Sprinkle with sesame seeds.
Top with tomatoes, sliced radishes and pickled onions.
Garnish with watercress, cilantro or basil leaves.
Drizzle with balsamic vinegar.
Note that you will have some leftover pickled onions. Refrigerate and save for up to a week to use as a burger or other sandwich topping.