Cut the pumpkin into wedges and arrange on foil. Sprinkle with 1 sprig of rosemary and thyme, salt and pepper to taste. Drizzle with oil, cover with extra foil and bake at 350F(180C) for 45 mins.
Meanwhile in a small bowl, whisk together 2 tbsp of olive oil, 1 sprig of thyme and the garlic. Season with salt and pepper.
Brush generously each Portobello mushroom with the oil marinade, on both sides. Allow to rest for 10 mins.
When the pumpkin is ready, remove the skin and transfer into a large bowl.
With the help of a potato masher, lightly mash until desired consistency and adjust with salt and pepper.
Heat up a non-stick pan to medium heat and cook the mushrooms on each side for 5-10 minutes.
Serve with pumpkin purée, caramelized walnuts* on top and watercress or baby leaves.
*For the Caramelized Walnuts ( you can also make them ahead, and store in an airtight container):
Heat a skillet on med-heat, add walnuts, sugar, walnut oil, ¼ tsp of salt and a pinch of pepper.
Cook for 5 mins, stirring occasionally until the sugar caramelizes.
Once ready allow to cool.