Combine cucumber and onion in a salad bowl. Season with salt and stir well.
Rinse purslane and pat dry with a tea towel. Pick off large leaves and tender upper part of the stems. Discard woody ends of the stems.
Add purslane to cucumber mixture along with feta cheese, olive oil, and vinegar. Toss well. Taste and season with additional vinegar or salt, if desired. Serve immediately.
