Preheat the oven to fan 180°C/gas 4. Line a baking tray with baking parchment.
Halve the large peppers lengthways and remove the core and seeds. Place cut side up on the baking tray, shaving a thin sliver off the curved underside to ensure they lie steady, if necessary.
Heat the olive oil in a non-stick sauté pan then add the red onion and cook over a medium heat for 5–10 minutes, until softened and just starting to colour. Add the garlic and diced pepper and cook for 2 minutes. Add the courgette and mushrooms, stir through and cook for 2 minutes.
Stir in the tomato purée and cook for 1 minute. Add the dried oregano, tomatoes and olives and cook for 3–4 minutes, or until the tomatoes soften.
Pour in the veg stock, then add the cauliflower ‘rice’ and stir through. Cook for about 5 minutes, until the liquid has reduced a bit. Stir through the basil and season with salt and pepper.
Spoon the mixture into the pepper halves. Top each with a slice of mozzarella and bake on the middle shelf of the oven for 20–25 minutes or until the cheese is melted and golden brown. Serve hot.