Pull the skin from the caps of the portobello mushrooms, and pop out the stalks.
Heat 3 tablespoons extra virgin olive oil in a large saucepan over a medium heat.
Lay the mushrooms in the pan, cap-side up, season with salt and pepper, and cook for 2-3 minutes.
Turn the mushrooms, season, and continue to cook. As they cook they’ll release their water content, intensifying the flavours of the mushrooms, and the roasted flavour is also essential.
Insert the tip of a knife into a mushroom and if it meets no resistance, the mushroom is cooked.
Transfer the mushrooms to a tray, leaving them to rest and release their juices.

For the duxelles filling:

Blend the chestnut mushrooms in a food processor (or with a handheld blender) to a puree.
Transfer the puree to a large saucepan and cook it over a medium-high heat.
Stir with a spatula, bringing the puree down from the sides of the pan. Allow the water content to evaporate –if you run the spatula down the middle of the puree and you can see the water content gather in the centre of the pan;
so continue to cook, reducing the mushrooms so that they become a paste, quite dry but intense in flavour. (Where possible reduce rapidly over a high heat to retain the freshness.)
If using port and Madeira, pour these in now and let them reduce by about 90 percent. Remove the pan from the heat, add the butter and work it in with the spatula. Season with salt.
Preheat the oven to 180°C / Fan 160°C / Gas Mark 4.

Make the herb crust:

In a food processor, or using a handheld blender, blitz the breadcrumbs, parsley and garlic to a fine powder. Add a splash of olive oil and work it through to create a paste.
Dab the rested portobello mushrooms with kitchen paper to remove excess juices and lay them, cap-side down, in a casserole dish.
Spoon the duxelles mixture into the cavity of each mushroom.
Sprinkle the herb crust paste over the mushrooms. Drizzle a little olive oil over the crust.
Bake for 15-20 mins.
Scatter with fresh herbs, season with sea salt and serve.