Mash the cauliflower with a fork or purée them, then add the butter, milk and spices to your own liking. To make it creamier use cashew/ soy milk. Lighter version: almond milk.
Fry the shallots in some oil or water, add your vegan patty, fry until the edges are a bit crispy, then add the mushrooms and let them cook until softened. Season with salt and pepper. Add veggie stock and milk. Then add mustard, soy sauce and fresh thyme leaves.
Mix cornstarch with some water until dissolved and add to the pan. Cook until sauce thickens.