Using a searing slow cooker (or hot pan on stove top) briefly cook chicken strips in hot oil in batches just to seal/change colour. Add the thin strips of onion in final batch of chicken sealing to soften them also.
Set slow cooker to LOW (with chicken and onion added to slow cooker if you had been sealing it on stove top)
Combine warm water, stock powder, sugar, fish sauce, sweet chilli sauce, soy sauce and tamarind paste and pour over chicken
Slow cook on low for 3hrs
Soak aside rice noodle in bowl of hot water now for 20mins
While noodles are soaking add cornflour slurry and bean sprouts to slow cooker and stir through
After 20mins has elapsed drain noodles, rinse, then gently stir through slow cooker mix to combine. Allow to cook for another 10mins then serve.
Drizzle with any remaining sauce and garnish with slices of green onion and fresh coriander
Note: You can purchase a jar of tamarind paste from the international cooking section of regular supermarkets