Add the guajillo and árbol chiles to a saucepan along with the onion. Cover with water and bring to a boil. Once boiling, turn off the heat, cover, and let sit for 20 minutes to soften.
Using a slotted spoon, transfer the softened chiles, onion, and garlic to a blender. Add 1 cup of the chile-soaking liquid, along with the oregano, cumin, and 1 tsp salt. Blend until smooth and set aside.
Pat the beef dry and toss with 1 ½ tsp salt, black pepper, and masa harina. Heat the oil in a large pot or Dutch oven over medium-high heat. Brown the beef until golden on all sides.
Pour in the blended chile sauce and 2 cups of beef broth. Stir to combine.
Bring to a boil, then reduce heat to low. Cover and simmer for 45 minutes, stirring occasionally, until the beef is fork-tender and the sauce has reduced and thickened, coating the meat evenly.
Taste and adjust salt as needed. If the sauce is too thick, add a bit more broth; if too thin, simmer uncovered for 10–15 minutes until reduced to your liking. Serve with warm corn tortillas and Mexican rice. Provecho!
