Heat oil in a large skillet over medium-high heat. Brown the beef cubes in batches, about 3-4 minutes per side. Transfer to slow cooker.
In the same skillet, sauté onions until softened, about 5 minutes. Add garlic and cook for 1 more minute. Transfer to slow cooker.
Add crushed tomatoes, beef broth, chili powder, cumin, smoked paprika, oregano, cayenne, salt, and pepper to the slow cooker. Stir well to combine.
Cover and cook on low for 8 hours or high for 4-5 hours, until beef is tender and falling apart.
About 30 minutes before serving, mix masa harina with ¼ cup water to make a paste. Stir into chili to thicken.
Serve with diced onions, shredded cheese, and crackers or cornbread on the side.