Make the pickled onions:Cut the onion into thin slices (I like to use a mandoline for this) and place into a large, heatproof bowl.In a small pot, add the vinegar, water, sugar, and salt. Bring the mixture to a boil and stir until the sugar and salt is completely dissolved. Let cool for a few minutes, then pour the mixture (it should still be hot) over the sliced onions. Let sit for 30 minutes at room temperature. Then the pickled onions are ready. Make the tuna salad:Drain the oil or water from the canned tuna. Place in a bowl and mix with the mayonnaise. Set aside. Assemble:Top each soda cracker with a spoonful of the tuna salad and some of the pickled onions. Add a dash of Tabasco. Eat and enjoy!