Heat vegetable oil in a frying pan and add curry leaves and mustard seeds until they crackle.
Add rhubarb, chillies, cumin, coriander, and palm sugar to the pan and stir well.
Cook for 3 minutes, then add lime juice and cook until the rhubarb softens.
Stir in mint leaves and remove the chutney from the heat.
Preheat the grill to high.
Place mackerel fillets skin-side up on a rack and grill for 5 minutes until crispy.
Toss watercress leaves with olive oil, salt, and pepper in a bowl.
Serve the fish on the watercress salad with the chutney on the side.