How to Make It
Prepare the Pickled Red Onions: Combine vinegar, water, sugar, and peppercorns in a medium saucepan. Bring to a boil over high, and cook, stirring occasionally, until sugar dissolves, about 2 minutes. Place onions in a medium heatproof bowl. Pour hot vinegar liquid over onions; cover and let stand until onions soften, at least 30 minutes. (Pickled onions may be stored in an airtight container in refrigerator up to 2 weeks.)
Prepare the Refried Black Beans: Heat oil in a medium saucepan over medium. Add garlic, cumin, and oregano; cook, stirring often, until fragrant, 1 to 2 minutes. Stir in beans. Add water, and bring to a boil. Mash bean mixture with a potato masher, and stir in lime juice and salt. Remove from heat; cover and keep warm.
Prepare the Cheese Sauce: Melt butter in a saucepan over medium, and cook until slightly browned, about 2 minutes. Remove from heat, and sprinkle with flour; whisk until well incorporated, about 1 minute. Very slowly drizzle in milk, whisking constantly, until mixture is emulsified and smooth, about 2 minutes. Return to heat, and cook over medium, whisking often, until thickened, about 5 minutes. Whisk in mustard, salt, and pepper. Gradually add cheese in small batches, whisking until melted and incorporated after each addition. Keep warm.
Assemble the Nachos: Drain Pickled Red Onions. Arrange half of tortilla chips and plantain chips on a serving platter. Dollop evenly with 1/2 cup of the Refried Black Beans, and drizzle with 1/2 cup of the Cheese Sauce. Top evenly with remaining tortilla chips and plantain chips. Dollop remaining Refried Black Beans over chips, and top evenly with pork, avocado, pickles, cilantro, and Pickled Red Onions. Drizzle with remaining Cheese Sauce. Serve with lime wedges.