Heat olive oil in a large saucepan over medium-high heat. Add the lamb in batches and cook, turning occasionally, for 5-7 mins or until well browned. Transfer to a heatproof bowl.
Add finely chopped Swiss Brown Mushrooms to the pan. Cook for 5 mins or until tender. Transfer to a bowl.
Add onion, carrot, celery and garlic to pan. Cook on low heat, stirring, for 5-10 mins or until onion softens, and just begins turning brown.
Add the wine and scrape pan to deglaze. Bring wine to the boil and reduce by half.
Return lamb and mushroom to pan.
Add Tomato Paste, Diced Tomatoes, Beef Stock, Bay Leaves, Thyme, Rosemary Sprigs and Brown Sugar.
Stir and bring to the boil. Reduce heat to low.
Cover pan and cook on the lowest simmer for 2-3 hours or until lamb is tender and falling apart.
Use tongs to transfer lamb to a heatproof bowl. Use 2 forks to coarsely shred.
Meanwhile, heat a heavy based frying pan, add butter. When it begins to foam, add sliced Portobello Mushrooms and fry on medium heat for 4-5 mins.
Return Lamb to tomato mixture and add Portobello Mushrooms.
Add Balsamic Vinegar and stir through.
Season with Salt and Pepper to taste.