Roughly chop the tomatoes, removing any tough central core. De-seed and roughly chop the pepper. Peel and roughly chop the cucumber, onion and garlic then place all in a blender.
Blend until smooth then stir in the vinegar, oil and salt. Taste and adjust seasoning to taste, with a little more salt, some pepper or a little more vinegar as you prefer. Refrigerate until ready to use.
I recommend English or Persian/Lebanese cucumber for this and try to find as good-quality tomatoes as possible for best flavor.