For the chutney, heat the oil in a non-reactive frying pan and add the curry leaves and mustard seeds. When they start to crackle, add the rhubarb, chillies, cumin, coriander and palm sugar, stirring well to combine. Cook for 2-3 minutes, then add the lime juice and cook for a further 2-3 minutes, or until the rhubarb is softened. Stir in the mint leaves, then remove the pan from the heat and set aside.
For the mackerel, preheat the grill to high.
Place the mackerel fillets skin-side up onto a rack and cook under the grill for 4-5 minutes, or until the skin is crisp and the fish cooked through.
To serve, place the watercress leaves into a bowl and toss with the extra virgin olive oil and season with salt and freshly ground black pepper. Divide the watercress equally among four serving plates, then arrange two mackerel fillets on top of each pile. Spoon the chutney alongside and serve.