Bring the stock to a simmer, next to where you will cook the risotto. Take a wide, heavy-bottomed pan or casserole, put half the butter in over a medium heat, and melt. Add the onion and sweat until it softens and becomes slightly translucent, about 5 minutes. Add the rice and stir with a wooden spoon until it is thoroughly coated in butter, about 2 minutes. Then take a soup ladle of hot stock and pour into the rice. Continue to cook and stir until this liquid addition is completely absorbed, about 3 minutes.
Repeat this procedure several times until the rice has swollen and is nearly tender. The rice should not be soft but neither should it be chalky. Taste and wait: if it is undercooked, it will leave a gritty, chalky residue in your mouth. Normally the rice is ready about 20 minutes after the first addition of stock. If it is taking much longer, the heat is too low or your rice has been hanging around in the store cupboard for too long.
Now the enrichment. Add the other half of the butter and half the Parmesan. Stir these in, season and cover. Leave to rest and swell a little more for 3 minutes. Serve immediately after this in soup plates, with more Parmesan offered separately.