Add enough water to a saucepan to reach a depth of 3cm. Bring to the boil over high heat. Place potato in a metal steaming basket. Place steamer in saucepan (make sure base of steamer doesn’t touch the water). Steam, covered, for 15 minutes or until tender.1
(It's best to steam rather than boil the potato. Boiling can saturate potato with water, which will increase the amount of flour required and result in a heavier gnocchi.) Transfer to a bowl and use a potato masher to mash until smooth. Season with salt and pepper.2
Add the egg and stir with a wooden spoon until combined. Add half the flour and parmesan and stir until combined.Add the remaining flour, in 2 more batches, until well combined and a firm dough forms. (If dough is too soft, add more flour.) Turn onto a lightly floured surface and knead until smooth. Line a baking tray with non-stick baking paper. Divide dough into 4 equal portions.3
Roll 1 portion into a 2cm-diameter log about 30cm long. Use a lightly floured knife to cut into 2cm pieces. Repeat with remaining dough portions.
Lightly flour your hands and roll each piece of dough into a ball. Use
your thumb to roll each ball over a floured fork. Place on tray.
Bring a saucepan of salted water to the boil over medium heat. Add one-quarter of gnocchi (drop onto base of pan in a single layer, but don't overcrowd as they can stick together) and cook for 3 minutes or until they rise to the surface. Use a slotted spoon to drain and transfer to a bowl. Cover with foil to keep warm. Repeat, in 3 more batches, with the remaining gnocchi.
To make the tomato & vegetable sauce, heat the oil in a large heavy-based saucepan over medium heat. Add the onion, carrot, zucchini, celery and garlic and cook, stirring often, for 5 minutes or until the vegetables soften.
Add tomato puree to the pan. Cover and bring to the boil
Reduce heat to low and cook, covered, for 10 minutes. Uncover and cook for a further 5 minutes or until the sauce reduces and thickens. Add Pasta seasoning and sugar. Stir until well combined. Season with salt and pepper.7
Divide the gnocchi among serving bowls. Spoon over the tomato & vegetable sauce, Sprinkle basil, Italian seasoning and parmesan to serve.