To make the dough, begin the night before. In a large bowl, stir together the flour, salt, and yeast until evenly combined. Add water, starting with just 3/4 cup, and stir until a sticky, shaggy ball of dough forms. The total amount of water needed will depend on the moisture level in your flour. Loosely cover the bowl with plastic wrap and let it sit at room temperature for 12-24 hours.
The next day, begin by preparing the caramelized onions. Thinly slice three onions and place them in a large skillet with 2 tablespoons of butter. Cook the onions and butter over low heat, stirring occasionally, for an hour, or until they are very soft and deep golden brown in color.
To make the flatbread, preheat the oven to 425 degrees. Cover a baking sheet with foil, then spread 1 Tablespoon of olive oil over the surface. Sprinkle the fermented dough with flour to prevent it from sticking to your hands, scrape it out of the bowl onto a floured surface, and flatten it into a disc. Continue to stretch and press the dough until it is a large rectangle, approximately 1/4 inch thick. It can be transferred onto the prepared baking sheet for easier stretching and pulling toward the end.
Spread the second tablespoon of olive oil over the surface of the dough, then sprinkle with 1/2 Tablespoon of Italian seasoning blend. Thaw the spinach in the microwave (about one minute on high), then squeeze out as much moisture as possible. Chop the artichoke hearts into bite-sized pieces.
Top the flatbread with the caramelized onions, spinach, chopped artichoke hearts, and crumbled feta. Bake for 18-20 minutes in the preheated oven, or until the edges of the crust are golden brown. Slice into 8-12 pieces and serve.
*Caramelized onions take about an hour to make, so it's often more efficient to make a large batch and freeze the leftovers. Caramelized onions can also be made using a slow cooker, and stored in the freezer until ready to use.**Leftover artichoke hearts freeze great, as well. Save them for your next flatbread!