• BEFORE YOU START: The night before, peel and thinly slice the onions. Place them in a large bowl filled with cold water, cover, and let them soak overnight. This step helps make the onions sweeter and milder. The next day, drain them very well before cooking.

Make the Dough

• Place the manitoba flour (or bread flour) and the semolina flour in a large mixing bowl. Make a well in the center and add the dry yeast, the sugar, the extra virgin olive oil and the mashed boiled potato.

  Start mixing the ingredients, gradually bringing the flour toward the center.
• Dissolve the salt in the lukewarm water. Add the water a little at a time while mixing. Do not pour it all in at once, because the amount of water can vary depending on the flour and the moisture of the potato.
• Transfer the dough on your work surface and knead it vigorously for about 10 minutes, until it becomes soft, smooth, and elastic. If the dough feels too dry, add a little more water. If it's too sticky to handle, add a small amount of flour. The final dough should be soft and slightly moist, but workable with your hands.
• Shape the dough into a ball and place it in a lightly oiled bowl. Cover with a clean kitchen towel or plastic wrap and let it rise in a warm place until doubled in size, about 2 hours.

Make the Filling

• Drain the sliced onions very well. Heat the extra virgin olive oil in a large skillet over medium-low heat. Add the onions and cook gently for about 30 minutes, stirring often, until they are soft and sweet.

  Do not let the onions brown too much. They should become tender and almost creamy.
• Add the pitted black olives, capers, cherry tomatoes, and chopped anchovies, if using. Season with a little salt, freshly ground black pepper and oregano. Be careful with the salt, because olives, capers, and anchovies are already quite savory.
• Cook for another 10 minutes, until the tomatoes soften and the filling becomes flavorful and well combined. Turn off the heat and let the filling cool slightly before using it.

Assemble the Stuffed Focaccia

• When the dough has doubled in size, transfer it to a lightly floured work surface. Divide it into two pieces: one larger piece, about two-thirds of the dough, and one smaller piece, about one-third.
• Roll out the larger piece into a disk large enough to cover the bottom and sides of a 26 cm (about 10-inch) round baking pan.
• Grease the baking pan generously with extra virgin olive oil. Place the larger disk of dough inside the pan, pressing it gently with your hands so that it covers the bottom and comes up the sides.
• Add the onion, olive, and tomato filling, spreading it evenly over the dough.
• Roll out the smaller piece into another disk and cover the filling with the second disk of dough.
• Seal the edges well by folding the excess dough from the bottom layer over the top layer. Press with your fingers to close the focaccia completely.
• Brush the top generously with extra virgin olive oil, spreading it over the entire surface with your hands or a pastry brush.
• Prick the surface with a fork to allow the steam to escape during baking.
• Bake in a preheated fan-assisted oven at 200°C (about 390°F) for about 30 minutes, or until the focaccia is golden brown on top. If using a conventional oven, bake at 220°C (about 425°F).
• Remove from the oven and let the stuffed focaccia rest for at least 10 to 15 minutes before slicing. It's delicious warm, but also excellent at room temperature.