Wash turnips and trim ends to make approximately 6 oz pieces. Heat a pan and drizzle half of the oil into the pan. Pan sear to brown the ends. Set aside.
Dice celery. Drizzle remaining oil into braising pan and saute onions, carrots and celery. Add the garlic and tomato paste. Lower heat and continue to cook. Stir until the past is caramelized, about 5 minutes. Add the wine to deglaze an cook until reduced by half. Add the rosemary, vegetable stock, diced tomatoes including juice, porcini powder, salt and freshly ground black pepper. Add browned turnips and braise until tender (about 2 hours) in a tightly covered pan in a 300°F oven or on the stove top over a very low heat. (If not tightly covered, braising liquid will evaporate and you need to add more stock.)
Make gremolata by toasting and chopping pistachios. Add parsley, lemon zest and garlic to pistachios and mix well. Add HALF amount of pistachio gremolata to the braise and stir well to incorporate. Serve turnip osso bucco with creamy polenta, top with braising liquid and garnish with remaining gremolata.
Notes
Porcini powder can be made with dried porcinis ground in a spice grinder / 1 oz ground = 1/2 cup.Homemade stock is a great way to use vegetable scraps to reduce waste. If you don't have homemade, you can substitute a purchased low sodium vegetable stock.