How to Make It
Pickle the onion In a medium bowl, whisk the vinegar with the lime juice, salt and sugar until the salt and sugar dissolve. Add the onion and refrigerate until bright pink and crisp, about 2 hours.
Make the salsa Preheat the oven to 400°. On a large rimmed baking sheet, roast the tomatoes, onion and garlic until softened and lightly charred, about 30 minutes. Let cool.
Meanwhile, in a medium bowl, cover the guajillos with boiling water and let stand until softened, about 15 minutes. Drain well and transfer to a blender. Add the tomatoes, onion, garlic, chipotles and epazote leaves, if using, and pulse until a chunky salsa forms. Transfer to a medium bowl and season generously with salt. Let cool completely.
Meanwhile, prepare the beans In a small saucepan, combine the beans, shallot, garlic and a generous pinch of salt. Add 2 cups of water and bring to a boil, then simmer over moderately low heat for 15 minutes. Drain, reserving 3 tablespoons of the cooking water; discard the shallot and garlic. In a food processor, combine the beans with the reserved cooking water and pulse until a chunky puree forms. Season the puree generously with salt.
Make the sandwiches Preheat the oven to 350°. In a large skillet, cook the chorizo in the olive oil over moderately high heat, breaking it up with a wooden spoon, until browned and cooked through, about 7 minutes.
Arrange the rolls cut side up on a large rimmed baking sheet. Spread each piece with some of the pureed beans and top with the chorizo and cheese. Bake for about 10 minutes, until the cheese is melted. Spoon some of the salsa on top and garnish with some of the drained pickled onions and micro cilantro. Serve right away, passing more salsa at the table.