Preheat oven to 180°C/360°F (160°C/320°F fan-forced).
Soften onions with olive oil in a Dutch oven on low heat, about 5 minutes.
Meanwhile, cut sausages up into small, bite-size pieces.
Add capsicum and garlic, bring heat up to medium-low. Cook for 2 - 3 minutes, stirring occasionally.
Add sausages and dried oregano, cook on high heat for about 3 minutes, stirring every minute to allow browning and caramelisation. Stir in worcestershire sauce, let it reduce for about 30 seconds.
Add passata, stock, dried pasta, sugar, salt and pepper. (note 2) Give it a mix and once the pot is boiling, transfer to the middle shelf of the oven. Bake for 15 minutes, no lid.
Crank the oven up to 210°C/410°F (190°C/370°F fan-forced) then take the pot out of the oven, scatter with mozzarella cheese. Bake lidless for a further 10 minutes.
To serve, sprinkle fresh basil leaves (optional but highly recommended).