Place cashews in a heat-resistant bowl. Bring three cups of water to a boil and pour them into the bowl with the cashews. Set a timer for 15 minutes.
Add olive oil to a skillet and saute the onions and garlic over medium heat until they are lightly caramelized. Place them in the blender along with the nutritional yeast, salt, black pepper and lemon juice.
When the cashews have soaked for 15 minutes, drain the water from the bowl and add the cashews and chicken broth to a high-speed blender. Puree until smooth. Taste and add additional salt, pepper, or lemon juice if desired.
Serve over fettuccine noodles and garnish with fresh parsley if desired.