Peel the carrots and parsnips, and remove the skins from the onions.
Pop onions, celery, carrot and parsnips into a food processor and chop finely (you can do this by hand, especially if you prefer chunkier veg- but I wanted mine to be small to thicken the sauce and appeal to the children!)
Coat the casserole steak with the flour, and season with salt and pepper. Crush the garlic cloves.
Heat the oil in a large, non-stick pan, and throw in the garlic. Cook for about 20 seconds, and then add in the beef. Stir around, and then leave to cook for about 3 minutes.
Stir again, and cook on a high heat for another 3 minutes.
Put the chopped veg into the slow cooker bowl, and add in the beef.
Mix together the mustard, worcestershire sauce, soy sauce, rosemary and thyme with a few tablespoons of the beef stock
Put the used beef pan back on the heat, and add the stock and sauce mix. Stir well, scraping away any residue left on the bottom of the pan.
Add the sauce into the slow cooker, stir well, and cook on high for 4-6 hours, or on low for 8-12 hours.
Delicious served with rosemary roast potatoes, broccoli and any other fresh green vegetables.