Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the oil in a frying pan, add the onion and fry, stirring, for 5 minutes or until softened and just beginning to turn golden.
Stir in the turmeric, chutney, stock, bay leaf and a little salt and pepper and bring to the boil. Pour into the slow cooker pot and add the rice. Add the smoked mackerel to the pot in a single layer. Cover with the lid and cook on low for 3–4 hours or until the rice is tender and has absorbed almost all the stock.
Stir in the peas, breaking up the fish into chunky pieces. Add extra hot stock if needed. Cook for 15 minutes more. Stir in the watercress or rocket, spoon on to plates and garnish with wedges of egg.