Preheat the oven to 200C/400F/Gas 6.
Place the sweet potato wedges on a baking tray and drizzle with the oil. Season with salt and freshly ground black pepper, then place into the oven for 15 minutes.
Remove from the oven and brush with the soy, ketchup and mustard.
Make the spiced beef. In a hot non-stick frying pan, heat the oil, then add the beef. Cook for 5 minutes until browned and completely cooked through.
Add the garlic and cook for a further 2 minutes. Finish by stirring in the chives, coriander and soy. Take off the heat.
Make the yoghurt dip. In a bowl mix the yoghurt with the garlic and mint, then season with salt and freshly ground black pepper.
To serve, place the sweet potato wedges on the plate and top with the spiced beef and a dollop of the yoghurt dip.