On a work surface, make a well with the flour. Pour in some of the warm water and crumble in the yeast, then add the olive oil and the teaspoon of sugar. Start mixing, gradually adding the rest of the water. Once the yeast is well incorporated, add the salt. Knead the dough until it forms a ball, then place it in a floured bowl and cover with plastic wrap.

Let the dough rise for at least two hours, preferably in the oven with the light on (I usually prepare the dough the night before for a longer rise, using only half the yeast).

Meanwhile, prepare the filling: Slice the leeks (or onions) and sauté them in a skillet with olive oil. While they cook, chop the tomatoes and add them to the leeks, then add the black olives, capers, and anchovies (if using). Season with salt and oregano, mix well, and cook over medium heat.

Once the dough has risen, place it on your work surface and divide it into two equal parts. Stretch the dough first with your hands, then with a rolling pin, until you have a sheet about 1/4 inch (about 1/2 cm) thick. Place it in a baking pan greased with olive oil.

Spread the filling evenly over the first layer, then cover with the remaining dough, sealing the edges. Prick the surface with a fork and brush with extra virgin olive oil.

Let the focaccia rest for at least an hour. Preheat the oven to 350°F (180°C), place the pan on the upper rack, and bake for about 40–50 minutes, or until golden brown.