Heat the barbecue grill or ridged fry pan, oil the shortloin with the cooking oil and set them to one side. Cut the chilies in half and remove the seeds. Finely slice the flesh and place in a mixing bowl with the lime juice, sugar, fish sauce, and sweet chili sauce.
Set to one side for 30 minutes for the ingredients to get to know each other and settle down. Roughly chop the coriander and mint, bruising it more than cutting it, and place in a mixing bowl. Slice the cucumber and carrot into long, matchstick sized lengths and add to the chopped herbs. Wash the watercress, pinch the leaves and branches from the stems. Cut the tomatoes in half and add to the salad with the watercress. Place the shortloin on the grill and cook until browned on the outside and medium rare on the inside, about 4 minutes on each side.
Remove to a warm place to rest for 10 minutes. Slice into strips 2 centimeters thick and add to the salad bowl. Pour the dressing over, toss gently together and serve while the venison is still warm. It's okay if the leaves wilt a little.