Heat butter or olive oil in a medium saucepan over medium-high heat. Add shallots and mushrooms (if using) and cook until softened, 5-7 minutes.
Add minced garlic and cook for 30 seconds until fragrant. Add the chopped cauliflower and chicken stock.
Bring the liquid to a boil, then reduce heat to a simmer, cover, and cook for 10-15 minutes, until cauliflower is very tender.
While cauliflower cooks, place raw cashews in a microwave-safe bowl, cover with water, and microwave on high for 3 minutes to soften. Drain and rinse.
In a high-speed blender, combine the softened cashews with 1/2 cup of water (or chicken stock). Blend until completely smooth and creamy.
Once cauliflower is tender, carefully pour the vegetable mixture into the blender with the cashew cream. Blend on low speed until totally smooth.
Pour the smooth sauce back into the saucepan. Stir in tamari and Parmesan cheese (if using). Season to taste with salt and plenty of freshly cracked pepper. Serve immediately over your favorite pasta!