Preheat the slow cooker if necessary; see the manufacturer's instructions. Finely chop the spring onions and herbs with a large knife or in a food processor if you have one. Mix with the lemon rind and juice, the oil, cumin seeds and a little salt and pepper.
Rinse the salmon with cold water, drain well and put on a long piece of kitchen foil, the width of the salmon. Press half the herb mixture over both sides of the salmon then use the foil to lower the fish into the slow cooker pot.
Bring the stock to the boil in a small saucepan, pour over the salmon and tuck the ends of the foil down if needed. Cover with the lid and cook on low for 1¾–2¼ hours or until the fish flakes into opaque pieces when pressed in the centre with a knife.
Lift the salmon out of the slow cooker pot using the foil and transfer to a serving plate. Mix the remaining uncooked herb mixture with the mayonnaise. Arrange the salad leaves on 4 plates. Cut the salmon into 4 pieces and place on the salad. Serve with spoonfuls of the herb mayonnaise.