In a small mason jar combine the red wine vinegar, olive oil, Dijon mustard and salt, put a lid on and shake till the vinaigrette emulsifies.
Using a mandolin or a sharp knife slice the cucumbers, tomatoes and red onions. Combine all ingredients in a salad bowl, top with fresh dill or another herb if using. Start by adding 2 tablespoons of the vinaigrette and tossing the salad gently, taking care not to tear the thinly sliced ingredients. Add more dressing if desired. Reserve the rest for another use. Serve immediately.
