01 - Preheat the oven to 400°F. Line a baking tray with parchment paper.
02 - Wipe the mushrooms clean and remove the stems. Place the mushroom caps, gill side up, on the prepared tray.
03 - Heat 2 tablespoons olive oil in a large skillet over medium heat. Add the onion and garlic; sauté for 2 minutes until softened.
04 - Stir in the chopped mushroom stems, carrot, and celery. Cook for 3-4 minutes until vegetables are softened.
05 - Add the ground beef and cook, stirring and breaking up lumps, until browned—about 5 minutes.
06 - Stir in tomato paste, oregano, thyme, Worcestershire sauce, and beef stock. Simmer for 3-4 minutes until the mixture thickens. Season with salt and pepper.
07 - Spoon the savoury mince mixture evenly into each mushroom cap, packing it gently.
08 - In a small bowl, mix breadcrumbs, parsley, and 1 tablespoon olive oil. Sprinkle the mixture over the stuffed mushrooms.
09 - Bake for 20-25 minutes until mushrooms are tender and the topping is golden brown.
10 - Serve warm, garnished with extra parsley if desired.