It's best to soak the cashews in water overnight. Alternatively, you can soak them in boiling water for 30-60 minutes before making the sauce.
In the meantime, preheat an oven or toaster oven to 400°F. Prepare your roasted garlic by slicing the top off and drizzling with some olive oil. Wrap it in tin foil and bake on 400°F for 25-30 minutes, or until the cloves are turning soft and browning.
Next, heat 1 tbsp olive oil in a saucepan on low-medium. Add the onions in and sauté them until they are browning.
As the onions and garlic are cooking, boil some water in a large pot. Once it has come to a boil, add a pinch of salt and fettuccine noodles. Cook them according to the package directions. Once the pasta is done, drain it and transfer back to the pot you cooked it in or to the pan for the onions. Set aside.
Once the garlic, onions, and cashews are ready, add all sauce ingredients to a blender and mix on high until a creamy texture is achieved. You may have to stop and scrape down the sides a few times, especially if you don't have a high-speed blender.
If you feel the sauce is too thick at this point, you can add in more plant milk 2 tbsp at a time.
Add alfredo sauce to the noodles and mix everything together. Serve this dish with some vegan parmesan, chopped parsley, and lemon zest on top!