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

Reduce the skillet heat to medium. Add the chopped onion and cook until softened, about 5 minutes. Add the minced garlic and cook for 1 more minute until fragrant. Stir in the chili powder, cumin, and oregano, and cook for 30 seconds to bloom the spices.

Transfer the onion and spice mixture to the slow cooker. Add the beef broth and tomato paste, stirring until everything is well combined.

Cover the slow cooker and cook on low for 6-8 hours or on high for 4-5 hours, until the beef is fork-tender.

In a small bowl, whisk the masa harina and warm water together to form a smooth slurry. Stir the slurry into the chili. Cover and cook on high for an additional 30 minutes, or until the chili has thickened. Season to taste with salt and pepper before serving.