Place the dried mushrooms in a bowl and cover with hot/boiling water. Let soak for 30 minutes while preparing the onions.
Heat a large skillet over medium-low heat, add the oil and the thinly sliced onions, salt and sugar and cook until very brown and caramelized, about 45 minutes. You’ll want to stir the onions fairly often so they don’t burn or stick.
Remove the stems from the mushrooms and thinly slice, then add to the cooked onions (along with a little more oil, if needed) and stir together. Drain the dried mushrooms (save the liquid) and finely chop, then add to the skillet. Stir, cooking often, until the mushrooms are reduced and browned. Stir in the balsamic vinegar and season to taste, adding more salt if needed. If the mushrooms start to stick, add a little of the reserved soaking liquid, then stir.
While the mushrooms/onions are cooking, place the cashews in the base of a blender with the water, a pinch of salt and the nutritional yeast. Puree until very creamy and smooth. Depending on the power of your blender, this may take a few minutes.
Bring a large pot of salted water to a boil and add in the fettuccine and cook until al dente. Drain, reserving 1 cup pasta water.
Add the cream to the skillet and reduce heat to low, then stir together. Add in the cooked fettuccine and toss together, adding reserved pasta water as needed to help the sauce cling to the noodles. Season to taste as needed, adding more salt and freshly cracked pepper as desired. Serve! This one is so good!
Cashews blend better when soaked. However, if you don’t have time or forget, then I recommend pouring boiling/hot water over the cashews and let soak for 10 minutes, then drain.