Heat 2 teaspoons of the oil in a small saucepan over medium heat until shimmering. Add the garlic and cook, stirring until lightly golden, about 1 minute. Add the beans, chipotle in adobo sauce, 1/2 cup water and 1/2 teaspoon salt. Bring to a boil then remove from the heat and set aside. Combine the onion, lime juice and a pinch of salt in a small bowl and set aside.
Position a rack in the center of the oven and preheat to 350 degrees F.
Stir together 2 1/2 cups lukewarm water and 1 1/2 teaspoons salt in a large bowl. Gradually add the corn meal into the water, using your fingers to stir and combine, until a soft and moist dough forms.
Divide the dough into 8 golf ball-size balls. Press each ball flat into an even 1/2-inch-thick patty.
Heat the remaining 1/4 cup oil in a large skillet over medium-low heat until shimmering. Working in batches, cook the corn patties until golden brown, about 7 minutes per side (making sure the oil is hot for each batch). Transfer to a cooling rack set on a rimmed baking sheet and bake until the edges are crisp and golden, about 10 minutes.
Add the cilantro and Cotija cheese to the onions and toss to combine.
When the arepas are cool enough to handle, use a thin serrated knife to slice a third of the way through each arepa. Then use the knife tip to cut open the center of each to form a pocket, taking care to keep the sides intact. Wipe down the knife between cuts to remove any sticky cornmeal. Stuff the arepas generously with slices of mango, avocado, the onion mixture and beans.
To make nice uniform patties, press the cornmeal balls between the tops of 2 plastic deli containers.
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