In a large bowl, whisk together all dressing ingredients until well-combined. If you want the dressing to be thinner, add water or milk 1 tbsp. at a time.
In the same bowl, add the tomatoes, cucumbers, and onion. Toss until everything is evenly-coated.
Top with parsley, serve, and enjoy!
Notes
For an extra refreshing salad, chill the sliced tomatoes, cucumbers, and red onion in the refrigerator for at least 30 minutes before assembling. This also helps the dressing adhere better.
Start with 1 tablespoon of water or milk when thinning the dressing, and add more gradually until you reach your desired consistency. A thicker dressing works well for coating.