Place tuna in a medium bowl, and break apart any large chunks with a fork.
Place the onion slices in a colander and rinse with cold water*. Add the onion, tomato, and avocado to the bowl with the tuna.
In a separate small bowl, mix olive oil, lime juice, cilantro, salt, pepper, and cayenne pepper. Pour dressing over salad, and mix gently.
*Rinsing the onion will remove its stinging bite and mellow the taste, making the onion sweeter. It will also help to make your eyes not water as the sulfates that irritate your eyes get rinsed away.
To Serve: place two corn cakes on a plate, and scoop some summer salad on top. Garnish with chopped green onion, cilantro, or sour cream, if desired. Enjoy!