In a large sauté pan over medium heat, warm the olive oil. Add the beef and brown, stirring occasionally and breaking up any lumps with the back of a wooden spoon, 7 to 10 minutes. Using a slotted spoon, transfer the meat to a slow cooker.
Add the garlic, cumin, oregano, chili powder, masa harina, stock, tomatoes, salt and pepper to the slow cooker and stir to combine. Cover and cook on high for 6 hours according to the manufacturer's instructions. Serves 8 to 10.