Finely dice the tomato, cucumber and pepper removing any tough stems and pepper seeds. Finely chop the parsley.
Put all of the above in a bowl with the oil, lemon juice salt and pepper (along with sumac, if using)and toss. Serve immediately and keep any leftover a day or two in the fridge.
Notes
You can optionally add a little red onion as well, if you like (no more than approx ¼ for this quantity) but it's also good without, as written.