https://somethewiser.com/2012/05/cornerstone-cooking-recipe-...He created 8
cornerstone dishes, which are recipes that can be a meal on day one but then the leftovers can be used for other dishes throughout the week.
So, my take is:
cornerstone resto = a resto that serves cornerstone dishes
cornerstone dish = a basic, fundamental dish that can be made into various other dishes although it is already great on its own.
If someone can summarize these into a compact phrase comprehensible by laymen and laywomen, that will be great.