In a medium bowl whisk together the mayonnaise, sour cream, red or white wine vinegar, Italian seasoning, olive oil, garlic, salt, pepper, and sugar.
For best results, cover and chill the dressing for at least one hour, and up to a full day before making the salad.
Thinly slice the cucumbers and red onions about ⅛-1/4 inch thick.
Mix the dressing together with the onion and cucumber. Serve immediately, or chill until ready to serve.
NOTE: This salad tastes best when the dressing is made ahead and chilled for at least one hour and up to 24 hours before serving. Once the salad is assembled, it can be chilled for another 1-2 days before serving if desired.