Preheat the oven to 180°C.
On a chopping board, cut off the hollow part of the butternut (chop this part up and freeze for later use) and leave the two solid parts of the butternut. Place the butternut on a roasting tray, drizzle 1 Tbsp olive oil over the butternut halves and roast uncovered for 20 minutes until tender. Remove from the heat and allow to cool.
In a frying pan, heat the rest of the oil and fry the onion, garlic and herbs together for 5 minutes on medium heat, then add the mushrooms and cook for 5 minutes until nicely browned.
Add the chopped morogo and dried cranberries and cook, closed, for 10 minutes. Season with herb salt. Allow the mixture to cool slightly; then once cooled, mix in one portion of the breadcrumbs.
To assemble, roll out the pastry on a floured surface, then sprinkle half of the leftover breadcrumbs in the middle of the pastry (an area where the butternuts will go; this will ensure that the bottom pastry stays crispy).
Spoon the cooled mushroom and morogo mix over the breadcrumbs, then place the roasted butternuts (side by side so the cut ends touch each other, creating one long butternut) over the mushroom mixture. Brush some herb pesto over the butternut and sprinkle the rest of the breadcrumbs over the top of the butternut.
Once the filling is in place, fold the pastry flaps to cover the butternut, joining them together at the top. Use your fingers to pinch together a pattern to enclose the butternut and keep the filling inside. Make incisions along the top of the pastry, allowing the Wellington to release steam during cooking. Brush some milk over the pastry to create a glaze.
Bake in the oven for 50 minutes, until pastry is golden and crispy.
Serve as a main with your favourite sides and salads.