- Mix the water, yeast, oil and semolina in a large mixing bowl and stir to blend. Add the flour once cup at a time, stirring well with a wooden spoon after each addition. Once the dough becomes too difficult to mix by hand, turn the dough out onto a lightly floured work surface and add the salt. Continue to knead until all the flour has been worked in (you may not need the final ½ cup of flour) and the dough develops and smooth and elastic consistency, about 10 minutes. Place the dough in a large, oiled bowl, cover the bowl with plastic wrap and let it rise until doubled in size, about 90 minutes.
- Turn the risen dough out onto a lightly floured surface again and divide it in half. Roll out each piece of dough into a rectangle 10 x 15-inches in size. Line two baking trays of this size (or a little larger) with parchment paper, and sprinkle each with a little semolina. Lift the rolled doughs onto each pan and cover the trays with a tea towel and let rise for 45 minutes.
- Remove the tea towels and use your fingertips to gently “dimple” the dough, cover and let rise another 45 minutes.
- Preheat the oven to 400°F. Toss the red onion, rosemary and olive oil to coat. Remove the tea towels from the dough and sprinkle the onions evenly over the dough, trying to coat as much of the dough with the olive oil. Sprinkle with sea salt. Bake the focaccia for about 25 minutes until a rich golden brown colour. Carefully remove the focaccia from the trays to cool (to prevent the bottom of the bread from going soft) and cool to room temperature before slicing.