Step 1: Heat olive oil in a large skillet over medium heat. Add chopped onion and sauté until softened, about 5-7 minutes.
Step 2: Add minced garlic and chopped mushrooms to the skillet. Cook, stirring occasionally, until mushrooms have released their liquid and are nicely browned, about 10-15 minutes.
Step 3: Stir in fresh thyme, balsamic vinegar, and soy sauce. Cook for another 2-3 minutes, allowing the liquid to evaporate. Remove from heat and let cool slightly.
Step 4: Transfer the mushroom mixture to a food processor. Add the soaked and drained cashews and nutritional yeast (if using).
Step 5: Pulse until a coarse pâté forms. Add 1 teaspoon of truffle oil. Process until smooth and creamy, scraping down the sides as needed. If the mixture is too thick, add a tablespoon of water or vegetable broth until desired consistency is reached.
Step 6: Taste and adjust seasoning with salt, pepper, and additional truffle oil if desired.
Step 7: Transfer the pâté to a serving dish. Cover and chill in the refrigerator for at least 1 hour to allow flavors to meld and the pâté to firm up.
Step 8: Garnish with fresh chopped parsley before serving.