Soak the dried porcini mushrooms in one cup boiling water for 25 minutes. Once rehydrated strain them out and chop them but make sure to reserve the liquid for later.
Toast the walnuts in a dry pan over medium to medium low heat. You want a little bit of golden coloring but no burns so watch your heat. This should take no more than three to five minutes.
Heat two tablespoons of olive oil in a large skillet over medium heat. Add the onion and cook for two to three minutes. Watch your heat so the onion doesn't burn.
Add both the cremini mushrooms, porcini mushrooms, onion powder and a pinch or two of salt and cook over medium or medium low heat for about 10 minutes. The liquid that the mushrooms expel will evaporate during this time and you will start to see some caramelization towards the end of 10 minutes. The goal is to gain more flavor from the caramelization without burning the mushrooms so watch your heat carefully and stir frequently.
Add the sage, a dash or two of black pepper and cook for another two to three minutes. Add another small pinch of salt if you feel it needs it. You want the mushrooms to be well-seasoned without being salty and keep in mind that when you blend the walnuts into the mixture, the seasoning will be toned down somewhat.
Add the reserved porcini soaking liquid and cook for another three to five minutes, scraping up the browned bits in the pan. Taste again for seasoning. (Tip: Don't stir the soaking liquid to keep the grit from the mushrooms at the bottom of the container. Collect the four tablespoons water from the top with a measuring spoon.)
Stir the walnuts, nutritional yeast and remaining two tablespoons olive oil into the mixture and transfer to a food processor. Add the white truffle oil if using and process until the mixture's texture is to your liking. I prefer a smooth pâté. (You can toss all of the ingredients into the food processor without stirring in the walnuts and nutritional yeast into the pan but I process in two batches). You shouldn't need to add anymore oil but if the mixture is too dry, add another tablespoon olive oil.
This recipe makes two cups pâté and you can store in one large ramekin or several smaller ones like I've done. For the vegan version, omit the butter topping I've added to one of my portions (pictured). If using the butter topping just spread the softened butter on top and smooth before chilling.