Boil 400ml of water, then pour into a jug or bowl. to the jug/bowl of water add the onion, garlic, stock cubes, dried herbs, mustard, Worcester sauce, salt and pepper. Mix well.
Add the beef to your slow cooker, then, pour the gravy mixture over the beef. Place the lid on the slow cooker and set to Slow cook (or High if your slow cooker doesn't have a Slow cook option). Pour yourself a wine or beer and put your feet up for 3 to 4 hours until the beef is cooked through and tender.
For the side dishes:
Slice the carrots and parsnips into quarters lengthways. Place them on an oven tray and pour over the honey and oil and mix well.
Roast the parsnips and carrots in the oven at 200 C / 180 C Fan / Gas 6 for 30 minutes.
Add the whole onions in their skins to the oven, on a separate oven tray, at the same time as the carrots and parsnips. Cook for the same length of time and temperature. Remove and peel once cool enough to handle.
Boil the peas and sweetcorn together for 10 minutes, then drain and keep warm.
Place the Yorkshire puddings in the oven for the last 10 minutes of the carrot and parsnip cooking time.
Serve the roasted vegetables, pea and sweetcorn mix, onions and Yorkshire puddings with the beef.
When you pour over the carrots and parsnips the honey and oil, place in the oven for two minutes then take them back out to mix around the tray. The heat softens the honey and makes it easier to mix.