Add all the mushroom mix ingredients (except the herbs) to a pan and cook over medium heat for 10 minutes until soft, stirring occasionally (if serving to babies under 12 months, blitz the veggies in a food processor to create a fine mince).
Then add all of the remaining ingredients and cook a further 3-5 minutes until it is thick and no remaining liquid is in the dish (it should be able to form and hold it’s shape).
Allow the mix to cool to room temperature before adding to the pastry. Once cool, preheat the over to 200c/390f.
Defrost the pastry by placing it on the bench for 5 minutes to soften, then add the filling to one edge and fold the other edge over the Wellington, gently tucking it underneath to seal.
Gently score the top of the Wellington however you like, adding optional decoration - repeat for one more Wellington if making medium sized ones, or if you are making a large wellington place the extra puff pastry over the top and seal.
Brush with soy milk, then cook in the oven for 40-45 minutes until the pastry is golden (if the decorations start to cook too fast compared to the rest of the pastry, just cover those with foil).
Allow to cool for 5 minutes before slicing. Enjoy!