In a pan, heat up the 2tbsp oil and cook the onions on a medium to medium-low heat until soft.
Add the sugar, salt, and pepper. Cook, whilst stirring to ensure it does not burn, until it darkens in colour and reduces in size – about 25 minutes. Set aside to cool.
Preheat the oven to 200°C and prepare 2 baking trays lined with baking paper.
Place pastry on a tray. Trim one end and reattach it to another end, by liberally pricking them together, if required. Score a 2-3cm thick border around the pastry. Do not cut all the way to the bottom. Prick all around inside the border.
Beat an egg and add a splash of water to loosen it up, then brush it all over the pastry.
Repeat steps 4-5 for the second pastry sheet.
(OPTIONAL step to make the base more sturdy) Bake the pastry for 10 minutes or until just slightly golden and puffed. Press the middle of the sheets down to deflate if necessary.
Drain the tins of Mutti Cherry Tomatoes (save the remaining liquid for another recipe).
To each sheet, layer on the onions then the tomatoes and mozzarella. Drizzle on a little olive oil and some salt and pepper, then bake again for 20 minutes or until the cherry tomatoes have burst. Cover the edges of the pastry with foil if you notice it getting too brown.
Serve hot with fresh basil and thyme and with a drizzle of balsamic vinegar.