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