Cut the ends off the ears of corn, but leave the shucks intact. Put all the ears in the microwave at once and cook on high for 9-10 minutes.
While the corn cooks put the olive oil, vinegar, Dijon mustard, salt and pepper in a medium-sized bowl. Whisk well. Add the tomatoes, cucumber, scallions and herbs.
When the corn is cool enough to handle, remove the shucks and silk. Cut the kernels from the cobs and add to the bowl. Toss. Fold in the feta. Serve immediately or refrigerated until needed.