Mustard Beef with Mushrooms

Name: Mustard Beef with Mushrooms
Prep time: 10 min
Cook time: 35 min
Total time: 45 min
Cuisine: American
Ingredients: 455g (1lb) of extra lean ground beef; 225g/8oz of fresh baby Portobello mushrooms (or chestnut mushrooms) - do not use frozen; 2 red onions, sliced finely; 3 cloves of garlic crushed; 400ml of beef stock and 80ml for browning mushrooms/onions (480ml/2cups in total); 1 tablespoon of wholegrain mustard; 1 tablespoon of arrowroot flour; pinch of dried thyme; 1 tablespoon of balsamic vinegar (good quality aged); 2 tablespoons of tomato puree (paste; salt and pepper to season; fresh chopped Italian parsley to serve