Toast the dried ancho and guajillo chiles in a dry skillet over medium heat for 30-60 seconds per side until fragrant. Place them in a bowl and cover with hot water. Let them soak for 30 minutes to soften.

Drain the chiles, reserving 1 cup of the soaking liquid. Remove stems and seeds. Add the chiles and reserved liquid to a blender and blend until a smooth paste forms. Set aside.

Pat the beef cubes dry and season generously with salt and pepper. Heat 1 tbsp of oil in a large skillet over medium-high heat. Working in batches, sear the beef on all sides until well-browned. Transfer the seared beef to the slow cooker.

Reduce the heat to medium, add the remaining 1 tbsp of oil to the skillet. Add the chopped onion and cook until softened, about 5-7 minutes. Add the minced garlic, chili powder, cumin, smoked paprika, and oregano. Cook for 1 minute, stirring constantly, until fragrant.

Pour the blended chile paste and the beef broth into the skillet. Bring to a simmer, scraping up any browned bits from the bottom of the pan. Pour this mixture over the beef in the slow cooker and stir to combine.

Cover the slow cooker and cook on low for 8 hours or on high for 4-5 hours, until the beef is tender.

In a small bowl, whisk the masa harina with 1/4 cup of warm water to create a slurry. Stir the slurry into the chili. Cover and cook on high for another 30 minutes to allow the chili to thicken.

Taste and adjust seasoning with salt and pepper as needed. Serve hot with your favorite toppings.