- Pre-heat oven to 400° F. Prepare baking sheet with parchment paper.
- Divide dough into 2 even portions. Roll each portion on a floured surface to 1/4-inch thickness. Make oblong shapes, not exceeding the length of your baking sheet. Place the 2 flatbreads next to each other on the baking sheet. Let stand at room temperature for 10 – 15 minutes to rest.
- Bake in pre-heated oven for 5 minutes. If you have any large bubbles, poke them several times with the point of a sharp knife. Bake another 5-7 minutes, until firm and starting to turn golden brown.
- While the flatbread is baking, add the butter and onions to a small saute pan over low-medium heat. Let the onions cook until softened and beginning to caramelize.
- Remove flatbreads to add toppings. Add the onions on an even layer on both flatbreads. Then add the apple slices. Top with a layer of cheese and sprinkle with walnuts.
- Return the flatbread to the oven for an additional 7-10 minutes until cheese has fully melted and walnuts have toasted.
- Allow to slightly cool, then top with arugula. Cut into triangles to serve.