Heat oil in a pan over medium heat and add onion. Brown the onion then add in rice and stir for a few minutes to coat and toast each grain slightly. Add in the wine and let it cook until evaporated. Add enough stock to cover rice by an inch. Cook it on medium heat, making sure to stir from time to time until most of the stock is absorbed. Repeat the above step but make sure to keep about 1/2 cup of the stock for the last step. Stir in the lemon juice and zest stirring it in. Add the last 1/2 cup of stock and butter, stir until both are completely absorbed. Stir in the Parmesan cheese, turn off the stove and cover. Allow it to sit a couple of minutes.