Combine vinegar, mustard, oil, salt and pepper in a small covered container and shake well to combine. Cut tomatoes into wedges and combine with onion, croutons and cheese. Add desired amount of dressing and toss to combine. Allow to sit at room temperature for about 20 minutes before serving for flavors to meld.
Recipe by Cooking With MicheleĀ® at https://cookingwithmichele.com/2009/06/heirloom-tomato-salad/