Cut the cucumber into 4 manageable chunks. One chunk at a time, place a towel over the cucumbers, and with one or two good whacks with a pan, smash the cucumbers. You just want the cucumbers to “pop”; you aren’t trying to pulverize them. Break the pieces into bite sized chunks, and then place them into a colander. Generously salt the cucumbers and toss them. Allow them to drain in the colander for about 10 minutes.
While the cucumbers sweat, make the dressing by whisking together the oil, vinegar, mustard, seasonings, and sugar.
To serve, toss the cucumbers, onions, peppers, sun dried tomatoes, and feta together with the dressing. Allow the flavors to mingle for about 20 minutes, or serve immediately.