Heat your oven to 200°C/Fan 180°C/Gas 6. Peel and cut the onion into thick wedges. Halve the tomatoes and cut each half into 4. Place onto a baking tray lined with foil. Drizzle over 1 tbsp olive oil and sprinkle in a pinch of salt and pepper. Set the tray to one side.
Peel and grate or crush the garlic clove. Place it in a roasting tin and mix in 1 tbsp oil. Slide into the oven for 5 mins to warm the oil. Take the tin out of the oven and tip the gnocchi in. Toss gently to coat the gnocchi in the garlicky oil. Season.
Return the gnocchi to the oven on the top shelf and slide the tray of veg onto the shelf below. Roast both for 25 mins, giving the veg and the gnocchi a good shake halfway through.
After 25 mins, take the roast tomatoes and onion out of the oven. Add half of them to a food processor with half the olives. Blitz with 2 tbsp water till puréed. This is your tapenade. If you don’t have a food processor, finely chop the olives then mash them into the roast tomatoes.
Take the gnocchi out of the oven. Add the tapenade to the gnocchi. Stir in the remaining roast tomatoes and onion and toss to mix. Tear the mozzarella and scatter over the dish. Slide back into the oven for 10 mins to melt the mozzarella.
Remove from the oven and scatter over the remaining olives. Tear over the basil leaves and add the rocket. Toss everything a few times to mix well. Serve the roast gnocchi and veg in warm bowls with a good grinding of black pepper.