Valencian Onion and Anchovy Flatbread (Coca de Cebolla y Anchoas)

1. Cook the onions

In a wide skillet, heat ¼ cup of olive oil over medium heat. Add the sliced onions and season them with salt and pepper. Allow the onions to cook slowly for about 40 minutes, stirring occasionally. This slow cooking process will develop a deep sweetness and a soft texture in the onions. You'll know they're ready when they are very soft and just beginning to color.

2. Prepare the dough

In a large bowl, beat the eggs with a fork. Add the olive oil and water to the eggs, followed by the salt and baking soda, mixing well. Gradually add the flour to the mixture, starting with the fork and then using your hands, until the dough becomes soft and smooth. Wrap the dough in plastic wrap and let it rest for 30 minutes.

3. Preheat the oven

Preheat your oven to 350°F.

4. Roll out and bake the dough

After the dough has rested, roll it out into a large, thin rectangle. Line a 12-by-16-inch baking sheet with the dough, pinching the edges to form a small rim. Brush the dough lightly with the beaten egg white. Bake the dough in the preheated oven for about 10 minutes, just until it becomes lightly firm. This step helps to seal the crust, preventing it from becoming soggy once the topping is added.

5. Add the topping and bake

Once the dough is slightly firm, remove it from the oven and spread the cooked onions evenly over the top. Arrange the anchovy fillets and olives decoratively over the onions. Return the flatbread to the oven and bake for an additional 30 minutes, or until the pastry around the edges is golden brown.