Preheat the oven to 325℉ (160℃).
Heat 1 tablespoon of the oil or butter over medium heat in a large lidded casserole that will go from the stove top to the oven (or use a frying pan for the stove top part and then transfer the mixture to a casserole before putting it in the oven).
Gently fry the onion in the oil or melted butter, stirring often, until it begins to soften and take on a little bit of colour. Remove the onion from the pan and keep warm.
Add the remaining tablespoon of oil or butter to the large casserole or frying pan.
Coat the beef cubes in flour (I use a Ziplock bag to do this, shaking the beef cubes in the flour until they are coated.)
Brown the beef cube, stirring frequently, for about ten minutes or until they are beginning to take on a bit of colour on all sides. Depending on the size of your pan, you may need to do this in batches. If so, keep the browned batches of meat warm with the onions until all the beef cubes have been browned.
Return the onions and all the browned beef cubes to the frying pan or casserole.
Add the beer, mixing it into the beef and onions thoroughly, scraping any bits off the bottom of the pan to mix into the stew.
Add ¾ cup of the beef stock and the mustard, mixing in thoroughly.
Carefully bring the mixture almost to the boil, stirring constantly so it does not stick.
If you are using a frying pan, transfer the mixture very carefully to an oven safe casserole now.
Tumble the carrots and parsnips into the casserole. Stir to coat with the liquid.
Tuck in the bay leaf and stir in the salt and pepper. (If your stock is very salty, go easier on the added salt.)
Cover the casserole and put it in the oven for an hour and a half, stirring every half hour.
Remove the casserole from the oven and remove the bay leaf.
Check the thickness of the gravy. If it has not thickened up, mix a tablespoon of cornflour with about a two tablespoons of water and stir into the casserole. If it is too thick, add a bit of the remaining stock.
Then return the casserole to the oven for another twenty to thirty minutes.
Check the stew for seasoning and add more salt and pepper if necessary.
Stir in the parsley.
Garnish with a bit more parsley if you like.