Heat oil in a cast iron or heavy bottomed saucepan on high. Dust pork with flour and sear in hot oil, in batches- so pork doesn’t stew. Remove from pan and set aide.
Add onions, garlic ,chilli and ginger. Sauté for a few minutes on medium -low until onions start softening.
Return pork to pan and, ketchup, white pepper, five-spice powder, tamari, the peel of one mandarin, vinegar and water. Mix well. Reduce heat to low, cover with lid and simmer gently for 1 hour. Add mandarin juice Season with salt and pepper. Garnish with spring onions and serve with rice