Slice cucumbers as thinly as you can. A mandolin or vegetable slicer will make quick work of it. Toss together the cucumbers and a large pinch of salt in a mesh strainer set over a bowl. Combine the feta, oregano, garlic and shallots in a separate bowl. Add the lemon juice and zest and whisk in the olive oil. Drain off and discard any water that has released from the cucumbers. Toss together the cucumbers and the dressing. Serve.