Into the work bowl of your food processor, place the onion and the garlic. Pulse until finely chopped. Set the chopped onion and garlic mixture aside. Place the portobellos in the food processor and pulse until finely chopped. Set mushrooms aside.
In a 12-inch skillet over medium-high heat, heat the olive oil until it shimmers. Place finely chopped onion and garlic mixture into the pan with the olive oil. Carefully stir the mixture and watch it carefully so the garlic doesn’t burn. Cook for 2 to 3 minutes or until the mixture starts to brown. Add the finely chopped mushrooms and cook, stirring often, until most of the liquid has evaporated from the mixture; this takes about 5 minutes.
Add the truffle salt, black pepper, dried thyme, tomatoes, and balsamic vinegar. Stir and reduce heat to low. Keep warm on the stovetop until ready to serve.
Bring a large pot of salted water to a boil. Spiralize the sweet potato using the largest set of blades for thick, hearty sweet potato noodles. Briefly blanch the sweet potato noodles in the salted water for 30 seconds. Remove noodles from water and place them into the skillet containing the Mushroom Bolognese Sauce. Toss to coat. Place mixture on a large platter and garnish with parsley leaves and shredded Asiago cheese. Drizzle the finished dish with a teaspoon of balsamic vinegar.