Put the potatoes in a large nonstick skillet and cover with water by 1/2 inch. Season with enough salt to taste like seawater. Bring to a boil and turn down to a simmer. Simmer the potatoes until mostly cooked through, 3 to 4 minutes. Drain and let cool on a plate in a single layer.
Rinse and dry the skillet; place over medium-high heat with 1 tablespoon oil. When the skillet is hot, add the chorizo and cook, breaking up with a wooden spoon, until crisp and brown, about 6 minutes,. Transfer to a plate with a slotted spoon. With about 1 tablespoon of fat left in the pan - remove some if there's too much or add more oil if there's too little - add the cooled potatoes and cook, stirring periodically, until golden brown all over, 4 to 6 minutes. Add the chorizo back to the pan and stir for about a minute. Remove from the heat and let cool completely.
Slice the bread loaf in half horizontally and lay the two halves cut-side up. Pull out some of the bread from both halves (reserve for another use), leaving a 1/4-inch border. Spread mayonnaise on both halves. On the bottom half, layer ingredients in this order: refried beans, chorizo and potato, salsa, pickled jalapenos, Cotija, tomato, cilantro, shredded lettuce, some salt and pepper, and the top half of bread. Wrap tightly in parchment, wax paper or plastic wrap and secure with twine if necessary. Press at least 3 hours with a brick (see Cook's Note) or cast-iron skillet on top to flatten the sandwich, refrigerated. About 45 minutes before serving, bring the sandwich to room temperature.
A brick that weighs about 4 pounds works best. Wrap it in foil to protect kitchen surfaces, if desired.
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