In a large mixing bowl, toss together the plantains and black beans with the oil, vinegar, coriander, and salt. Spread the mixture out onto a baking sheet lined with parchment paper.
Bake for 16-20 minutes, flipping the plantains after 8 minutes, so they brown evenly. Your plantains are done when they're browned on the outside and tender on the inside. Cooking time will vary depending on how ripe your plantains are. If they don't look done at 16 minutes, check them at 18 minutes. If they still don't look totally done, flip them, and do 2 more minutes.
Make the quick pickled onions while the plantains and beans bake.
Stuff the onions into a pint-sized mason jar. Combine the remaining ingredients in a small saucepan, and heat them until they boil, stirring until the sugar dissolves. This should only take a few minutes. Turn off the stove, and pour the hot brine into your jar. The heat from the brine will soften the onions.
Screw the lid onto your jar loosely, and set it on the counter to cool. Once it’s cool enough to handle, you can tighten the lid and stick the jar into the fridge.
Assemble the tacos.
Divide your tortillas between four plates. Top each one with ¼ of the plantain mixture, ¼ of the sliced avocado, and a tablespoon or two of the pickled onions. Devour!
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Notes
Your plantain should be almost all brown and very soft. If it were a banana, you'd say it was no good, but instead, it's a perfect plantain!
A red or white onion is fine. You want about 2 cups of sliced onion total.