Coat a large sautoir abundantly with olive oil. Add the onions and season generously with salt. Bring the pot to medium-high heat. Cook the onions, stirring frequently until they are very soft and aromatic but have no color (7-10 minutes). Don't rush this - you don't want crunchy onions in your risotto!
Add the rice and stir to coat with the olive oil. Cook the rice for 2-3 minutes to toast, stirring frequently.
Add the wine to cover the surface of the rice and stir frequently until it has completely absorbed (you will start to feel the rice sticking to the bottom of the pan).
Add the reserved mushroom water and then add chicken stock until the liquid ahs covered the surface of the rice. Stir frequently until the stock has absorbed into the rice. Repeat this process 2 more times. Check for seasoning - you will need to add more salt!
During the third addition of stock, add the reserved sauteed mushrooms and 2 tablespoons of the pureed porcini mushrooms.
When the stock has absorbed into the rice and the rice is cooked but still al dente, remove the pot from the heat.
Add the butter and cheese and whip until well combined. This will set the perfect consistence of the rice. The rice should flow and not be able to hold its shape and look very creamy. Add the chives and serve immediately.