Heat the vegan butter (or oil) in a large pan over medium heat. Sauté the onions until translucent and browned, about 4-5 minutes. Add garlic and sauté for 1 minute longer.
Add mushrooms and fry until lightly browned, about 5 minutes. Season with salt and pepper to taste.
Pour in vegetable broth, soy sauce, and plant-based cream.
In a small cup, add cornstarch to a little water and stir to dissolve. Stir in the sauce and bring to a simmer. Cook for 5-7 minutes, until gravy has thickened to desired consistency. Add nutritional yeast flakes (optional) and more salt and pepper if needed.
Top your mashed potatoes with mushroom gravy, fresh herbs and toasted pine nuts (optional). Serve immediately.