A couple hours before assembling the salad, mix the dressing so it has time for the flavors to blend. Do so by placing the mayo, sour cream and dried ranch seasoning in a bowl and mix ot combine. Cover with plastic wrap and set aside in fridge.
Salad assembly:
Cook the bacon on a skillet over medium low heat until crisp, drain off excess fat.
Cut romaine in half and place on plate. Sprinkle tomatoes, bacon and onions over lettuce. Top with ranch dressing as desired and croutons.