Marinade chicken in turmeric powder and a pinch of salt, and set aside.
Blend onion, tomatoes and lemongrass to a smooth fine paste, adding a little water if necessary.
Heat oil in a non-stick wok and fry the marinated chicken till lightly browned. Set aside.
In the same pan, add in garlic, ginger and sambal olek, and fry to a few seconds till fragrant. Slowly add in the onion-tomato-lemongrass paste, stirring continuously till it starts to become slightly darker in colour and give out oil from the sides.
Mix in the fried chicken, and some water if necessary depending on the sauce consistency you prefer. Season with salt and let it cook covered for another 5-6 minutes till the chicken is completely cooked through. Serve warm.