Generously coat the bottom of a large nonstick skillet with the oil and place it over medium heat. You'll want the oil to be about ⅛ inch deep. Once the oil begins to shimmer, add plantain slices in an even layer. Let the plantain slices cook for three to four minutes, until they're browned on the bottoms. Use a spatula to gently loosen them from the skillet as they cook, as they have a tendency to stick.
Flip the plantain slices and cook them for another three to four minutes, until browned on both sides. Transfer them to a paper-towel lined plate when they're done. Continue cooking the plantain slices in batches, adding oil to the skillet as needed.
Once all of the plantain slices are cooked, you can either wipe out the excess oil in the skillet if there's a lot of it, or add some if it seems dry. You want a light coating on the bottom. Place the skillet over medium-high heat and add the onion. Cook the onion for about four minutes, stirring often, until softened and browned.
Add the garlic, cumin, and smoked paprika to the skillet. Cook everything for about a minute, stirring constantly, until the mixture becomes very fragrant.
Stir in the beans. Continue cooking the mixture for another one to two minutes, just to heat up the beans.
Remove the skillet from heat. Stir in the lime juice and cilantro. Season the bean mixture with salt and pepper to taste.
To assemble the tacos, slather each tortilla with guacamole, then divide the bean mixture and plantain slices among the tortillas.
Serve.