In a large saucepan, sauté onion in oil until soft and golden.

Add minced garlic and stir, sautéing for several additional minutes, until fragrant.

Add sliced mushrooms to the pan and sauté over medium heat for 10 to 15 minutes

or until soft.

Add salt, teriyaki sauce, sesame oil, red wine vinegar, and brown sugar. Mix well and bring to a boil. Lower heat and simmer for an additional five to 10 minutes.

Remove from heat. Blend sauce with an immersion blender or in a food processor, using the S blade, until smooth.

Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius).

Wash roast and pat dry. Sprinkle with onion soup mix, truffle salt, and freshly ground black pepper. Drizzle lightly with olive oil and rub seasonings into the meat.

Place in a deep baking pan and pour blended sauce over the top. Add the whole mushrooms to the pan. Cover tightly and bake for about three and a half to four hours, checking for doneness with a fork. A fork should slide in easily, and the meat should feel tender when done.

Allow meat to cool to room temperature or refrigerate until firm before slicing. Reheat the sliced meat in its gravy and serve by spooning the sauce over the meat and topping with the whole cooked mushrooms from the pan.