Place the PAN, water, oil, and salt in a mixing bowl and stir until the mixture just comes together.
Cover with a damp towel and allow to rest for 15 to 20 minutes.
After the dough has rested, mix it with your hands to remove any lumps.
Taking a small amount (about ⅓ cup) of the dough, form a flat and round disc, even in thickness.
Brush each side of the disc with a small amount of vegetable oil and place onto a sheet pan lined with parchment paper.
Place a cast iron skillet (or some sort of heavy bottom skillet) over medium-high heat and brush with a small amount of vegetable oil.
Once the skillet is hot, place a few arepas onto the surface, making sure not to overcrowd, and cook for 7 to 10 minutes on each side, until they are golden brown on each side and make a hollow sound when lightly tapped (turn the heat down to medium if the pan gets too hot).
Remove them from the heat and repeat with the remaining discs (you can store the cooked arepas, on a prepared sheet pan, in a warm oven while you cook the remaining batches).
For the guasacaca: Place the avocado, onion, garlic, serranos, vinegar, and lime juice into a blender and blender until smooth. Add the cilantro, salt and oil and continue to blend until smooth. Season with more salt and pepper.
To assemble: Open the arepas down the side to create pocket.
Open the pocket and fill with ½ cup of carnitas and top with a generous drizzle of guasacaca and a sprinkle of cotija. Repeat with the remaining arepas and serve.