In a deep bowl, mix corn flour and salt. Add warm water little by little. Using your hands, start to mix, keeping a steady circular motion while mixing.
Mix until you have a homogeneous mixture that does not stick to your hands. With your hands, divide dough into equal balled portions then press between palms of your hands to create a flat patty or "arepa" shape.
Place arepas in a pan or flat non-stick skillet at medium heat and cook for approximately four minutes on each side. Cook until they are golden on each side. Set aside.
In a medium sauté pan, add olive oil and onion and cook until translucent. Add tuna and sauté for 3-4 minutes.
Transfer the sauté to a bowl and allow it to cool briefly. Once cooled, add avocado, lemon juice, cilantro, yogurt, and salt and pepper to taste.
Top arepas with salad or, if you prepared larger arepas, stuff them with the mixture.