Slow braised beef short ribs with cheesy mash

• 2 kg/4 pounds beef shortribs (I prefer using bone-in short ribs cut into smaller pieces but you can use larger pieces as well)
• 2 onions finely chopped
• 3 carrots peeled and finely chopped
• 2 celery sticks finely chopped
• 6 garlic cloves finely chopped
• 2 tbsp flour (optional)
• 2 pieces fresh Rosemary
• 1 bay leaf
• 1 cup tomato puree/passata
• 2 cups beef stock
• 2 cups red wine
• 500 g/1 pound mushrooms
• salt and pepper to taste

For the cheesy mash

• 1 kg/2 pounds potatoes peeled and quartered
• 2 tbsp butter
• ½ cup milk
• 1 cup grated cheddar cheese
• salt and pepper to taste