Make the mushroom rub: Strip thyme and rosemary leaves, then blend or finely chop with stock cube. Mix with marmite, maple syrup, olive oil, and boiling water until thickened. Set aside.
Prepare the Portobello mushrooms: Remove stalks, place caps on a foil-lined baking sheet, and brush with rub. Roast at 200ºC for 30-40 minutes, then cool. Stack mushrooms tightly on skewers.
Make the duxelle: Pulse chestnut mushrooms and portobello stalks. Sauté shallots, garlic, mushrooms, and thyme in butter until moisture evaporates. Season and cool.
Make crepes: Blend ingredients, cook in a hot pan, and set aside.
Assemble the wellington: Lay crepes on cling film, top with duxelle, then place the mushroom stack. Wrap tightly and freeze for 1-2 hours.
Prepare pastry: Roll out on floured surface, wrap mushroom filling, seal edges, and chill for 30 minutes to 2 hours.
Bake: Preheat oven to 200ºC. Brush pastry with glaze and bake for 35 minutes until golden. Rest for 5 minutes before serving.
