If you haven't already prepared your arepas, which take about 30 minutes (not included in prep time), do that now.
Preheat oven to 400 degrees F (209 C). Peel your plantains and slice on a diagonal into 1/2-inch pieces. Then add to a parchment-lined baking sheet and toss with oil. Arrange into an even layer and bake for about 15-20 minutes or until golden brown and caramelized. Toss near the 10-minute point to ensure even baking.
In the meantime, add (slightly drained) black beans to a small saucepan and heat over medium heat until bubbly and hot. Season with cumin and salt and stir to combine. Then turn heat off and set aside (cover to keep warm).
Lastly, prepare guacamole by mashing avocado in a small mixing bowl and adding lime, salt, onion, and cilantro. Stir to combine, and taste and adjust flavor as needed. Add more salt for saltiness, lime for acidity, or onion for crunch. Set aside.
You're ready to serve. Slice your hot arepas in half (or cut a "pita pocket" and stuff). Add guacamole, beans, plantains, and any other desired toppings, such as Habanero Hot Sauce, cabbage, or cilantro!
Best when fresh. Store leftovers (separate from each other) in the refrigerator up to 3 days.
*Nutrition information is a rough estimate for 1 of 6 sandwiches (as the recipe is written) with all of the guacamole, beans, and plantains and no optional toppings.