The Food Network's Iron Chefs have nothing on the chefs that spend their days in the kitchens of area kitchens. And they're not afraid to put their culinary skills to the test. Every Tuesday night this spring, Harley Smith, executive chef of 10 Center in Newburyport, is inviting guest chefs from the region to a face-to-face cook-off. Last week, Kevin Carney of Cassis Bistro Francais in Andover accepted Smith's challenge. The two chefs spent the day in 10 Center's kitchen, each preparing a three-course meal built around three key ingredients that they served to diners that night.