Prepare the Cabbage: Slice half a cabbage very thin, wash with water, drain well, add salt, lemon juice, cover with water, stir well, and reserve.
Cook the Potatoes: Add potatoes to water, bring to a boil, and cook covered for 45 minutes until very soft. Then let cool.
Prepare Potato Filling: Cut the cooled potatoes into pieces with the skin on, mash with salt, add cornstarch, and mix well.
Cook Filling: Fry chopped onion in hot oil until lightly browned, add garlic and chipotle pepper, fry briefly, add mashed potato, and stir to integrate. Let cool.
Make the Sauce: Roast tomatoes and serrano pepper, sauté onion and garlic in hot oil, add chiles de árbol briefly, then blend all with a little water and salt.
Assemble and Fry Tacos: Fill tortillas with potato mixture, add cheese slice if using, fold and fry in hot oil until browned on both sides. Drain on absorbent paper.
Serve: Serve tacos with shredded cabbage, tomato, roasted chile de árbol sauce, and crumbled cheese.