Heat oven to 170C/150C fan/gas 3. Put a large non-stick, flameproof casserole dish on a high heat and add 1 tbsp oil. Season the beef and brown in batches until well coloured on both sides, making sure you don’t overcrowd the dish. Remove each batch and set aside on a plate.
Add the remaining oil to the pan and lower the heat to medium. Toss in the onions and cook for about 10 mins until softened and golden brown. Stir in the flour and cook, stirring, for 1-2 mins.
Add the stout and stir well to deglaze the pan, scraping up any sticky bits. Return the beef to the pan, pour in the stock and add the thyme. Bring to the boil, then put the lid on and cook for 1½ hrs.
Add the mushrooms to the casserole, stir well and return to the oven for a further 1½ hrs.
Season to taste and stir in half the chopped parsley. Scatter over the remaining parsley to finish and serve with mash, if you like.