The classic topping for this comforting Alsace speciality is a mixture of fresh soft cheese and sour cream mixed with finely sliced onions, topped with Bacon lardons. If you want to add cheese, try something like Swiss emmental and chunks of stinky munster.
Click here for this recipe: https://www.goodfood.com.au/recipes/tarte-flambee-cheese-onion-and-bacon-flatbread-20180522-h10ejn