Preheat the oven to 200°C. Place the short ribs in a large roasting dish and roast until well browned on both sides. Remove from the oven and set aside.
In a large casserole dish over medium to high heat, sauté the onions, carrots and leeks in a small amount of olive oil until soft and the onions are translucent. Add the browned short ribs along with the roasting juices, stock, tomatoes and sugar. Turn the oven down to 150°C, cover the casserole with a lid and place in the oven to slow-cook for 4 hours. Remove and season well with salt and pepper. Scoop off any excess oil and keep warm over low heat on the stove top while the gnocchi is being prepared.
Cut a small cross in the top of each potato, place on a baking tray and bake at 180°C for 40 to 60 minutes until crisp on the outside and soft on the inside. Bring a large pot of salted water to a gentle simmer over medium to high heat. Remove and discard the potato skins and pass the hot potatoes through a ricer, and then press through a fine sieve into a large bowl. Add the remaining ingredients and mix to form a soft, elastic dough. Lightly flour your surface and hands, and roll the gnocchi into long cylinders 1cm in diameter. Cut into 2cm pieces and roll each piece gently with the back of a fork to shape the gnocchi. Add to the simmering water and cook for 2 minutes or until risen to the surface. Remove, drain and toss lightly in olive oil.
To serve: stir the fresh herbs into the ragu and serve warm with the gnocchi.