Polenta’s familiar corn-gold color here becomes spinach-green and tomato-red. The polenta is divided into two batches, one mixed with sautéed chopped spinach, the other with a tomato sauce. Sandwiched between the red and green layers is ground pork cooked with wine and pine nuts. Twenty or so minutes in the oven meld all parts into an unusual torta, one of the best-looking things one can do with polenta.