Melt 1 tbsp. of the margarine in a saute pan over low heat. Add the shallot, leak and garlic and saute for about 5 minutes. Add the button mushrooms and half of the thyme leaves.
Cook until the mushroom liquid has evaporated and the mixture is just moist, about 10 minutes. Transfer the mixture to a bowl.
Melt the remaining margarine in the pan over low heat and add the shitake mushrooms. Cook 5 minutes and transfer to the bowl with the rest of the cooked mixture. Add the chives and the black pepper and give it all a good stir.
Next, place your portabella caps upside down on a greased baking sheet and brush with mustard. Add the rest of the thyme leaves and some black pepper.
Divide your mushroom mixture and put it in each of the portabella mushroom caps.
Cut the puff pastry in either 4 rounds or squares, which ever you prefer and put it over the mixture in the portabella mushroom. You can brush the top of the puff pastry with some vegan margarine or if you prefer an egg wash.
Bake for about 15 minutes. Just check them to see if they are golden brown on top. Some ovens are hotter then others. Serve warm.