In a medium bowl, add the steak, season with salt and pepper, and let rest while preparing the other ingredients.
Place the prepared guajillo chiles in a medium saucepan and cover with water.
Bring to a boil, then reduce the heat and simmer for 10–15 minutes, until the chiles are soft and tender. Remove from the heat and let soak while preparing the other ingredients.
Place the potatoes and ½ teaspoon salt in a medium saucepan and add cold water to cover by 1 inch.
Bring to a boil over medium-high heat, then reduce the heat to medium-low and simmer, stirring once or twice, until the potatoes are tender, about 15 minutes. Drain, transfer to a medium bowl, and set aside.
In a blender, add the beans with some broth and blend until smooth. Set aside.
Heat 2 tablespoons of oil in a 12-inch nonstick skillet over high heat until hot. Add the onion and garlic and stir-fry until lightly golden, about 3 minutes.
Add the halved and whole serrano chiles and stir-fry, turning them so they blister and char slightly on all sides.
When the onion and garlic are lightly browned, and the chiles are well charred, remove everything and place it in the blender with the hydrated guajillo chiles and ¼ cup of their soaking water. Blend until smooth, strain, and set aside.
In the same skillet over medium-high heat, cook the chorizo, breaking it up with a wooden spoon, until well browned, 4 to 6 minutes. Lower to medium heat. Add the cooked potatoes and mash lightly.
Add the blended beans and the strained sauce. Taste and adjust seasoning with salt if needed. Cook, stirring, over medium-low heat until thick, about 12 minutes.
Transfer to a medium bowl and set aside. Wash the skillet and wipe it dry with paper towels.
Heat 2 tablespoons of oil in the now-empty skillet over medium-high heat until just smoking. Sear the beef with the garlic, stirring occasionally. Add water to create steam. Cover and cook until the liquid evaporates.
Once the liquid evaporates, add the onion and stir-fry for 5 minutes. Transfer to a medium bowl and set aside. Wash the skillet and wipe it dry with paper towels.
Adjust the oven rack to the middle position and heat the oven to 400°F (200°C).
To assemble, warm the tortillas. Line a sheet pan with paper towels and set aside. Place the filling in the center, add a piece of cheese, and fold.
Heat the remaining 4 tablespoons of oil in the clean skillet over medium-high heat until shimmering. Arrange 2 tacos in the skillet with the open sides facing away from you.
Cook, adjusting the heat so the oil actively sizzles and bubbles appear around the edges of the tacos, until crispy and deeply browned on one side, about 2 to 3 minutes.
Using tongs and a thin spatula, carefully flip the tacos and cook until deeply browned on the second side, 2 to 3 minutes, adjusting the heat as necessary.
Remove the skillet from the heat and transfer the tacos to the prepared wire rack. Blot the tops of the tacos with a double layer of paper towels.
Place the sheet with the fried tacos in the oven to keep warm. Return the skillet to medium-high heat and cook the remaining tacos. Serve immediately with meat stuffed into each taco and with the chiles toreados.