Remove stems and seeds from the guajillos. Briefly toast in a dry skillet over medium heat until aromatic (15–30 seconds per side). Transfer to a bowl and cover with hot water; let soak for 15 minutes until pliable. Reserve some soaking liquid.

Roast 3–4 Roma tomatoes and 1 clove of garlic until blistered and softened. Drain one soaked guajillo and blend it with the roasted tomatoes, roasted garlic, a pinch of salt, and a bit of the chile soaking liquid to loosen. Taste and adjust salt. Set aside.

Pat the tenderloin dry and rub all over with 2 tsp kosher salt. Heat 1–2 tablespoons of oil in a heavy pot over medium-high heat. Brown the pork on all sides until a golden crust forms, about 6–8 minutes total. Remove and set aside.

In the same pot, add the quartered white onion and sauté for 3–4 minutes to soften. Return the pork to the pot, add the remaining soaked guajillo (whole or torn), reserved garlic cloves, peppercorns, and bay leaf. Pour in enough water to come halfway up the pork. Bring to a gentle simmer, cover partially, and cook for 1½–2 hours until the pork is fork-tender.

Remove the pork to a cutting board. Skim fat from braising liquid and discard bay leaf and peppercorns. Shred the pork with two forks. Add a cup of the braising liquid plus 3–4 tablespoons of the prepared salsa to the shredded pork in the pot. Simmer for 5–10 minutes to marry flavors and moisten the meat. Taste and season with salt as needed.

Heat refried beans in a small saucepan over low heat, stirring until creamy and hot.

Thinly slice ¼ white onion, crumble queso fresco, shred lettuce, and warm tostada shells in a 350°F oven until crisp (about 5–8 minutes).

To assemble, spread warm refried beans on each tostada, then top with shredded pork, drizzle with guajillo salsa, and add lettuce, raw onion slices, and queso fresco. Serve immediately for the best crunch.