Heat the oil in a large saucepan over a medium heat. Peel and finely chop the onion and garlic. Finely chop the celery.
Put the onion and celery in the saucepan and cook for 2 minutes. Add the garlic and cook for another minute.
Tip in the passata, chicken stock and mixed herbs. Season with salt and pepper. Bring to the boil then simmer for 20 minutes.
Meanwhile, make the stuffing. In a bowl, whisk together the mascarpone, goats cheese, chives and egg until smooth. Season with salt and pepper.
Slice the mushrooms. Place them over the base of a large ovenproof baking dish. Sprinkle the garlic powder over the top.
Pour the hot sauce over the mushrooms.
Stuff the pasta shells with your filling one by one using a teaspoon. Put the shells stuffing side up in the sauce, trying to make sure they are submerged as much as possible.
Sprinkle the mozzarella over the top. Cover the dish with foil and put in the oven for an hour. Remove the foil and cook for another 10-15 minutes until golden brown. Serve immediately.