Preheat oven to 425F. Spray two round 9-inch baking dishes (see notes) with non-stick spray, and sprinkle with cornmeal. The cornmeal will help prevent the flatbread from sticking and is optional, but it also adds a bit of nice texture at the bottom.
In a bowl, combine the flour, Greek yogurt, and garlic powder. Carefully mix with a fork until the dough is in rough, shaggy pieces, and then use your hands to fully combine the dough. If it is very sticky and wet, add a tiny bit of flour at a time. If all the flour isn't combining, add a teaspoon of yogurt at a time until the dough is well combined and not sticky - it might not ever get super smooth but that is OK for this recipe. Knead the dough together for 3-4 minutes.
Divide the dough in half and roll each half into a ~10 inch circle flatbread. This doesn't have to be perfect, the a rustic look works well in this recipe! If the dough sticks to the rolling pin, add a tiny bit of flour, but try not to add too much otherwise the dough will get tough.
Place each rolled out flatbread into the prepared pan and add the toppings: Crumble half the feta cheese over each flatbread. Sprinkle a little less than half the mozzarella over each flatbread (save a bit for later). Add a layer of spinach. Arrange the roasted red pepper and artichoke pieces all over the flatbread, and sprinkle with more mozzarella cheese. Season to taste with freshly ground black pepper and salt.
Bake at 425F for 12-15 minutes, or until the flatbread is nicely browned and crisp around the edges, and the cheese has melted. Let the flatbread cool for a few minutes prior to slicing and serving.
You can make your own self rising flour by adding 1 1/2 teaspoons baking powder and 1/4 teaspoon salt to 1 cup of all purpose flour. So that would be 2 1/4 teaspoons baking powder and 1/4 + 1/8 teaspoons salt per 1.5 cups flour for this recipe. Source: King Arthur Flour
Or just buy self rising flour (here is a link) since it's not any more expensive than regular flour
Instead of two 9-inch pizzas, you can bake this as one large pizza - just roll the dough out into a free-form rustic-shaped pizza and place it on a large baking dish
If you want to add extra spinach, double the spinach and microwave it for a few seconds so it wilts and you can fit more on the flatbreads before baking