Avocado-Tuna Sandwich

Slice the onion into thin rings and place in a tall bowl. In a small saucepan, bring apple cider vinegar and water to a boil. Once the vinegar boils, pour it over the onion and refrigerate for at least 30 minutes.

After the specified time, mash the avocado with salt and pepper using a fork. Then finely chop the cilantro and mix it with the avocado. Slice the jalapeño pepper into thin rings and set aside for now. Spread the avocado mixture on the bread, add tuna, pickled onion (amount to taste), and jalapeño, and sprinkle everything with sesame seeds.