For the the flatbread:

1. Whisk together warm water and yeast in a large mixing bowl, or the bowl of a stand mixer. Let sit for 10 minutes to give the yeast a chance to bubble. Add in flour, salt, and olive oil. Stir vigorously by hand using a wooden spoon for 6-8 minutes, until the dough forms a ball and becomes elastic. It will pull away from the spoon, but remain a bit sticky. If using a stand mixer, attach the bread hook to the mixer and mix on low for 5-6 minutes. Next, cover the dough with plastic wrap and let it sit at room temperature for 30 minutes to lightly ferment.
2. Meanwhile, fire up the grill or, if baking, preheat the oven to 400 degrees fahrenheit.
3. While the grill heats up, lightly dust a baking sheet with flour, then transfer the dough to a working surface also dusted with flour. Cut the dough into two equal pieces. Flour a rolling pin then roll out the pieces until they just under 1/4″ thick. Transfer them to the baking sheet.
4. Brush the first flatbread with olive oil and place it on the hot grill with the oiled side down. Grill for 1-2 minutes on each side, carefully checking with tongs to make sure it doesn’t burn. Once both sides have been grilled, remove from heat, and repeat with the second flatbread.
5. If baking, disregard brushing with olive oil; just slide the baking sheet into the oven and bake for about 25 minutes. The bread will be soft and chewy on the inside with a crisp and toasted exterior.

For the toppings:

1. Preheat oven to 400 degrees fahrenheit. Place the grapes on a parchment paper-lined baking sheet and brush them with balsamic vinegar. Transfer to the oven and cook for 15 minutes, or until the grapes begin to shrivel and wrinkle.
2. Toast the walnuts in the oven at 400 degrees fahrenheit for about 5 minutes. This can be done while the grapes are baking.
3. Meanwhile, stir together blue cheese and cream cheese. Spread each cooled flatbread with cheese, then add roasted grapes, toasted walnuts, and top with fresh arugula.