Beef cheeks in red wine

Name: Beef cheeks in red wine
Ingredients: 1kg beef cheeks, cut into 3-4 pieces; 4 tbsp olive oil; ½ celery stick, finely diced; 4 carrots, finely diced; 2 onions, finely chopped; 2 garlic cloves, finely chopped; 2 parsley sprigs, finely chopped, plus 1 tbsp; 1 bay leaf; 5 peppercorns; 5 allspice berries; 1 thyme sprig, plus extra for garnishing; 1 rosemary sprig; 2 tbsp tomato purée; 1l good-quality red wine; 700ml beef stock; 1 tbsp granulated sugar (optional); 4 tbsp cold butter, diced; 12 medium-sized mushrooms; mashed potatoes, to serve