Place the tomatoes in a strainer set over a bowl or the sink. Toss with a good pinch of salt and let excess liquid drain from them. Add the cucumbers to the strainer, adding another pinch of salt.
Place the onions in a large bowl. Add the tomatoes and cucumbers and toss to combine. Add a few turns of pepper, a drizzle of olive oil, a squeeze of lemon and the feta. Toss to combine. Taste and adjust as desired. This will keep covered in the fridge for 2 days.