Mix the yeast, sugar and water in a jug and leave to rest until foamy (approximately 10 minutes).
To work by hand: Sift the semolina, bicarbonate of soda and salt onto a clean work surface and make a well in the middle. Pour the yeast liquid and lemon juice into the well. Using a fork, mix the liquid, gradually bringing in the flour from the sides. Continue to mix with a fork until just combined. Use your hands to bring the dough together, then turn onto a clean, lightly floured work surface. Add the olive oil and knead the dough until smooth and elastic. It should come away cleanly from the work surface and takes approximately 10 minutes.
If using a mixer: Combine all ingredients for the Schiacciata dough into a mixer. Mix on the second speed until the dough is smooth and elastic (approximately 5 minutes). It should pull away from the sides of the bowl.
Preheat the oven to 230°C conventional oven/ 210°C fan-forced (450° Fahrenheit/Gas 8). Adjust metal racks to the lowest groove of the oven. Line 2 large shallow baking trays with non-stick baking paper and grease generously with canola oil.
Lightly dust the inside of a large, clean bowl with flour (large enough to hold dough that is double in size). Place the dough inside the bowl and rub extra virgin olive oil on top. Cover with a damp tea towel and leave to rest in a warm place for 2 hours or until the dough has doubled in size.
To make the filling, heat the olive oil in a large non-stick frying pan over medium heat. Add the spring onions (scallions) and cook, stirring for 3 minutes or until soft. Add the potatoes, then break up the sausage meat with your hands and add to the frying pan. Stir the capers and basil into the potato and sausage mixture. Cook until the potatoes are golden and sausage is cooked through. Season with salt and pepper to taste. Use a slotted spoon to transfer the potato and sausage mixture to a plate lined with paper towel. Set aside to cool for 15 minutes.
Remove the dough from the bowl and punch with your fist to push the air out. Knead gently on a lightly floured surface. Divide the dough into 2, leaving the unused portion covered by the damp tea towel. Use a rolling pin to roll each piece of dough into a rectangle shape that is 5mm thick.
Arrange half of the potato and sausage filling over half of one rectangle, overlapping slightly and leaving a 1cm border around the edge. Dot tablespoons of half of the Italian passata (or Italian canned chopped tomatoes) over the potato and sausage filling. Sprinkle half of the Caciocavallo and Parmigiano Reggiano cheese over the potato and sausage filling and add a drizzle of extra virgin olive oil. Brush the outer edge with water, then fold the dough over to enclose the filling. Press edges together to seal, then crimp edges. Gently transfer the Schiacciata to the greased baking tray. Use a fork to prick the top of the Schiacciata and brush with olive oil. Repeat with the remaining portion of Schiacciata dough.
Reduce oven temperature to 200°C conventional oven/ 180°C fan-forced (400° Fahrenheit/Gas 6). Bake until the top and base of the Schiacciata are browned (approximately 40-50 minutes).
Slice and serve immediately or prepare in advance and reheat – buon appetito.
Use a good quality, fruity extra virgin olive oil.