If poaching the chicken, add the chicken breast, sliced onion, salt, and black pepper to a pot and cover with water by an inch or two. Bring it to a boil, then lower the heat and simmer for 10-15 minutes until the chicken is cooked through. Remove the chicken and set aside to cool.
In a large bowl, mash the avocados and add the garlic, bell pepper, onion, cilantro, mayonnaise, and lime juice. Shred the chicken and add it to the bowl. Season with salt and black pepper to taste. Cover and keep it in the fridge.
Arepas
In a large bowl, combine the arepa flour and salt. Add the water and stir until combined. Knead until a smooth dough forms and let it rest for a couple of minutes to let the arepa flour hydrate and thicken.
Separate the dough into eight portions and roll them into balls. Flatten each ball into 3.5in/10cm disks. Cook on a griddle or skillet over medium heat until golden on both sides, about 5-7min per side.
Take the arepas off the heat and slice them. Fill with as much filling as you like.