Instructions
Play through the game until you reach level seven to unlock Bart's Exchange.
Go to Costolot's General Store in town. Purchase at least one turnip. Turnips cost 50 coins each.
Go to Bart's Exchange. Purchase a "Tinker Ticket." A bronze ticket is 15 coins and has a 60 percent chance of being successful. A silver ticket costs 52 coins and has an 85 percent chance of being successful. A gold ticket costs 300 coins but it is guaranteed to work.
Talk to Bart and he will ask which item you would like to transform. Select "Turnip" from the list of items. If the turnip is transformed successfully it will change into stew.