Caramelized Onion and Tomato Tart Au Gratin | Can't Stay Out of the Kitchen | this delectable tart is filled with #tomatoes, 4 cheeses & #CaramelizedOnions. It is so mouthwatering. Good as a #SideDish or for #Breakfast. #TomatoTart #TomatoPie #RomanoCheese #ParmesanCheese #AsiagoCheese #FontinaCheese #Holiday #HolidaySideDish #HolidayBreakfast #Easter #MothersDay