Cook the pasta according to the packet instructions, then rinse it under cold water until it is completely cooled. Leave it to drain in a sieve.
Meanwhile, mix the ingredients for the dressing and season to taste with salt and freshly ground pepper.
Combine the pasta, cucumber, red onion, peas and feta in a large bowl and toss well with the dressing.Serve straight away, or keep the pasta salad in the fridge until ready to serve.