I found this recipe for Creamy Pesto Salad Dressing at Cooking Classy. It is a copycat based on the dressing from The Old Spaghetti Factory. I have never been to The Old Spaghetti Factory so I cannot vouch for any similarities, but it is a great dressing nonetheless. You can substitute milk for buttermilk, just add a squeeze of lemon juice. As long as your ingredients are fresh, this dressing will keep in the refrigerator for about a week.