Shrimp & Toasted Pumpkinseed Tart

This recipe is based on the famous Mexican sauce pipian verde, made from toasted pumpkinseeds, chiles, and sour cream. I learned about it from my friend chef Roberto Santibañez, former culinary director of the Rosa Mexicano restaurant group. Pipianes (the plural) are a fascinating category of Mexican sauces, used frequently in Mexico, but not as well known in the United States, as mole sauces. This one uses fairly hot serrano chiles to flavor