Pasta with Yogurt, Spinach, and Sweet Onions

This recipe used to be our go-to for entertaining vegetarians -- back when vegetarians were, you know, a rare breed. Now, thankfully, the dish has moved into our regular dinner rotation. The hardest part about it is securing the sheep's milk yogurt -- but not really, you can find it at Whole Foods or even a slightly-gourmet supermarket -- then it's jus…