Slow cooker beef brisket

Name: Slow cooker beef brisket
Ingredients: 1.2kg brisket rolled; 2 tbsp vegetable oil plus extra for drizzling; 2 large onions sliced; 4 tbsp plain flour; 500-550ml golden ale; 1 tbsp yeast or beef extract; 1 tbsp dark brown soft sugar plus extra to taste; 1 tbsp balsamic vinegar plus extra to taste; 1⁄2 bunch of thyme; 2 bay leaves; 500ml hot beef stock; 2 tsp cornflour (optional); 2kg Maris Piper or other floury potatoes chopped; 4 tbsp horseradish; 100g butter; 100g crème fraîche; handful of parsley chopped