Cook the corn cobs in a large saucepan of salted boiling water for 5 minutes until just cooked through. Drain and cool under cold running water. Using a sharp knife, slice the kernels away from the cobs.
Mix together the lime rind and juice, chilli and cumin in a large bowl, then stir in the oil. Season well. Add the corn kernels, black beans, tomatoes and spring onions and gently toss together with the lettuce leaves. Scatter over the feta and coriander to serve.