This Southwestern-influenced casserole is sometimes called tamale pie, but there is really nothing pie-like about it, and the corn base is closer to polenta and grits than tamales. Many people shy away from experimenting with tomattillos, which look like small green tomatoes (the two are not related), but they are wonderful in this recipe; in fact, this is a great recipe for anyone who hasn't used tomatillos before. The casserole can be made 1 or 2 days in advance before baking.