Whisk all ingredients for marinade (EXCEPT curry and ginger powder) in a bowl until well combined. Pour half of the marinade over the sirloin strips and mix well. Keep refrigerated to marinate overnight. Add curry powder and ginger powder to the remaining marinade and mix well. Keep refrigerated and use as salad dressing. Preheat 1 tbsp. cooking oil in a large frying pan until very hot. Add in marinated beef strips and stir-fry until medium doneness, for about 1 minute. Combine salad greens, bell pepper strips, sliced water chestnuts and stir-fried beef in a large salad bowl, pour over the dressing and toss well.