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Heat oven to 190C/fan 170C/gas 5. Heat 1 tbsp oil in a large pan, then fry mushrooms for 4-5 mins, stirring occasionally, until lightly browned. Remove with a slotted spoon, then set aside. Add remaining oil to the pan, then fry the celery, onion and garlic over a low heat, stirring occasionally, for 5-6 mins. Add wine, stock and thyme and bring to the boil. Simmer for 30 mins.
Add the port and redcurrant jelly. Bring back to the boil, then boil for 10 mins until reduced by half and thick and syrupy. Season to taste, discard the thyme and stir in the mushrooms. Simmer for 5 mins.
Meanwhile, pull the tops off the brioche and reserve, then pick out dough from the centre to leave a thick shell. (Process remaining dough to make breadcrumbs, then freeze for future use.) Place rolls on a baking sheet, then bake for 5-7 mins until hot and crisp.
Spoon the mushroom mixture into the brioche rolls, then garnish with the flat-leaf parsley. Serve immediately with any remaining gravy, potato wedges and green beans.