Place 2 ¼ cups of warm water in a mixing bowl. Then add the salt, and a tablespoon of oil, stir. Add the precooked corn flour, stirring constantly with a spoon. Knead until you obtain a dough with a nice smooth consistency, without any lumps. If the dough is dry you can add the extra water, one tablespoon at a time and mix again.The arepa dough should come off the bowl easily.
Let the arepa dough rest for 5 minutes.
Divide the dough into 8 to 10 balls of the same size and flatten them between your hands to form the arepas into a flat patty or disk shape.
Add the remaining tablespoon of oil to a frying pan or a griddle pan, make sure the pan is evenly coated with the oil.
Turn the stove on to medium-high heat, and add the arepas to the heated pan. Make sure there is enough space in between them (to be able to turn them easily). When the first five minutes have passed, turn them over and cook them covered for another five minutes on the opposite side. Remove the arepas from the pan and let them rest. Repeat if working in batches.
For the tuna avocado salad filling:
Prepare the avocado tuna filling while the arepas are cooking. You can also prepare it ahead of time and have it ready to stuff the arepas.
Sprinkle ½ of the lime juice and a sprinkle of salt over the chopped white onions.
Save about ¼ of the avocados, to slice and decorate the arepas when serving; you can sprinkle a little bit of lime juice on the slices to keep them from darkening. Mash the rest of the ripe avocados in a large bowl.
Add the tuna fish, olive oil, lime marinated onions, mayonnaise, chopped cilantro, crushed garlic (if using), the rest of the lime juice, and salt to the bowl with the mashed avocado.
Mix well. Taste and adjust salt, mayonnaise or lime juice if necessary. Save
Use a sharp knife to carefully open the warm arepas lengthwise and fill each one with the tuna avocado salad. After filling the arepas with the salad, add the extra diced or sliced avocado. Serve immediately.