Preheat the oven to about 325℉ (or about 170℃ - 160℃ for a fan oven).
You need a large oven safe casserole with a lid that you can also use on the stove top. (I use a Le Creuset casserole.) Alternatively, you can use a large frying pan for the first part, and transfer everything to a casserole with a lid before you put it in the oven.
Heat the olive oil over medium heat in an oven safe casserole with a lid that you can also use on the stove top.
Sauté the onion until it begins to look translucent.
Add the beef and brown it in the oniony oil.
When the beef has begun to brown, stir in the barley and let it get coated in the oil too.
Now add the beer, the stock, the Worcester sauce and the tomato puree and stir though.
Stir in the mushrooms, if using.
Tip in the vegetables and stir gently.
Stir through the salt, pepper and thyme, and then tuck the bay leaf into the stew, covering it with the liquid.
Put the casserole in the oven for an hour and a half, stirring carefully every 30 minutes to avoid breaking up the bay leaf.
You may need to add more stock as barley absorbs liquid differently every time. The amount of liquid you need will also be affected by what type of vegetables you use.
Remove the casserole from the oven, and take out the bay leaf.
Serve in warmed bowls.