Wash the baby spinach and place it on a clean kitchen towel. Cover with another towel and press down to absorb some of the moisture. Heat a large pan (high sides are good here) over low to medium heat and add the spinach, stirring gently until it is wilted (around 2 minutes) Place a sieve or colander inside a bowl (make sure the bottom of the colander sits above the bottom of the bowl) and transfer the spinach inside. Set aside to cool and drain off some of the liquid.
Using the same pan, turn the heat down to low and add one tablespoon of olive oil. Add the sliced onions with a half teaspoon of salt and cook, stirring regularly, for around 10 minutes or until soft and beginning to caramelise. Add the brandy and continue cooking for another 5 minutes or until the brandy is cooked off (evaportated) and the onions are soft and shiny.
Remove the onions from the pan and transfer them to a large bowl.
Again using the same pan, turn the heat up to medium and add the mushrooms (don't add oil at this stage). Cook the mushrooms until they begin to colour (around 3 minutes). Add the remaining tablespoon of oil, the chopped thyme and rosemary, the remaining salt and pepper and the garlic and cook for another 3-4 minutes or until the mushrooms are cooked through and any moisture has cooked away. Transfer the mushrooms to the same bowl as the onions.
To the mushrooms and onions add the chopped apricots, crumble in the chestnuts or add the pecans and the panko bread crumbs. Squeeze any moisture from the spinach and add that to the bowl too.
Combine the mixture together - use your hands to really get in there and combine it well. For best results allow your filling to cool to room temperature. Preheat oven to 205°C (400°F).
While the filling is cooling, remove the puff pastry from the freezer and leave it at room temperature to thaw for around 30 minutes.
Lay a sheet of puff pastry down on a lined tray. Spoon the filling in the centre of the pasry and, with clean hands, form it in to a tight log. Bring the bottom of the pastry up to the centre of the filling and then bring the top section to meet it. Press down gently and brush with a little water to seal.
Like wrapping a present, fold the sides up to close your Wellington roll. Brush with a little water to ensure the pastry is well sealed. Gently flip the roll over so the seam is on the underside.
Brush the pastry with dairy-free milk and lightly score the top with a sharp knife. Sprinkle with course salt and pop the Wellington in the oven for 30 minutes or until the pastry is crispy and golden brown.
Remove from the oven and transfer to a serving plate. Slice to serve.