Cook the peas in a saucepan of boiling water for 3 minutes. Alternatively, cook them in the microwave for 1½ minutes on full power.
Flake the sardines into chunks and put them in a large salad bowl with their sauce. Rinse and drain the lentils, dice the cucumber and chop the onion. Add the lentils, peas, cucumber and onion to the sardines. Add the mint to the salad with the lemon rind and juice and a little pepper and toss together.
Separate the lettuce into leaves and arrange them on serving plates. Spoon the sardine salad on top and serve.