Rehydrate the porcini mushrooms in a small bowl, with just enough boiling water to cover them. Leave them for 20 minutes and drain, retaining the liquid.
Meanwhile, heat 1tbsp oil in a large pan over a medium heat. Add the onion and garlic and cook until the onion is translucent, about 5 minutes.
Next, add the rice, sliced chestnut mushrooms and rehydrated porcinis. Stir and cook for a couple of minutes to combine and toast the rice.
Add the wine and porcini water to the pan. Bring to a simmer and wait for this to absorb, stirring occasionally.
Keeping the stock at a simmer in a saucepan, add it to the risotto pan a ladle at a time. Stir often, waiting for the last ladle of stock to be absorbed before adding the next.
After around 15 minutes, all of the liquid should be absorbed and the rice should have no chalky texture inside.*
Add nutritional yeast and season to taste. Stir well. Add the olive oil and give it one final stir before serving.
Notes
*If the rice is still chalky at this point, continue adding stock or boiling water and stirring until rice is cooked through.