Heat the oil in a large, heavy-based frying pan over a medium heat. Add the ginger and chili flakes, stir well and cook for 2-3 minutes, making sure the it doesn't burn.
Season the beef all over with pepper, then sprinkle over the five-spice powder and mix to coat evenly. Add to the pan and fry for 3-4 minutes, turning the beef regularly until lightly browned all over.
Transfer to the slow-cooker. Pour over the stock and stir in the gluten free soy sauce. Cover and cook on high for 4 hours, until the beef is meltingly tender.
When the beef is cooked, whisk the cornflour with 1 tablespoon water to a smooth paste, stir in the maple syrup.
Add to the slow-cooker, cover and cook for 12-15 minutes, or until the sauce is glossy and has thickened slightly.
Serve the beef with steamed rice or boiled rice noodles and stir-fried vegetables.