The term "casserole" referes to the style of cooking as well as the cooking pot itself. A casserole is a covered earthenware dish in which foods are cooked. The secret of all these kinds of dishes is long, long cooking times at low oven temperatures.
So, obviously, you need a suitable dish. I prefer a proper earthenware one, will a well-fitting lid and a small hole for steam. Of course, there are all sorts of other kinds, including cast iron and Pyrex.
I like to use good-quality stewing steak, cut into chunks about 3/4 inch on a side. A little over a pound makes a decent meal for four people. Note that none of the ingredients are fried before putting in the casserole - recipe books often refer to "browning" the meat in a little fat beforehand - but I find that the meat is more tender if it is not cooked separately.
Wash the steak and put it in the casserole dish. Slice coarsely (this means big pieces, and nothing to do with swearing!) a large onion, and chuck that in too. Peel a couple of carrots and slice into rounds (or use whole baby carrots), and add these to the pot. You might try other root vegetables - baby turnips work well - or perhaps a few mushrooms, washed but otherwise left whole.
The meat and vegetables need to be completely covered by liquid. You could use a beef stock cube dissolved in boiling water. I also like to add a little "something extra" - a splash of sherry or red wine perhaps, or a larger quantity of dark beer (not lager!). I find bottled or canned Guinness works well, and then of course you'll have to drink the rest of the bottle!
Now, the seasoning. Classically, this dish would be flavoured with a bouquet garni (usually a combination of parsley, thyme and bay leaf) - these days, its easy to use the herb "teabags", so that the larger herbs can be removed before the dish is served. Alternatively, you could just add ground herbs: coriander and bay leaf makes a good combination. Finally add lots of freshly ground black pepper and a little salt.
Cover the dish carefully, and place in an oven at 180C (Gas Mark 5) for a couple of hours. You could cook this for even longer at lower temperatures.
This dish needs thickening to make a really nice gravy. These days, I use cornflour, since Tracey is allergic to wheat. After a couple of hours, remove the casserole from the oven, and add a couple of heaped teaspoons of cornflour already stirred into half a cup of cold water. Stir in the thickening, and return the casserole to the oven for another half-hour. (This is the time you should be putting on the potatoes to boil.)
Serve the casserole straight from the pot at the table, with lashings of mashed potatoes to soak up the gravy.