Hi J - blame the changes in language -
A 'stew' is the name of a dish traditionaly boiled to cook, containing meats and veg.
A 'casserole' is the name of the dish in which a stew would be made. That the casserole was then placed in an oven led to the meal being called a casserole. Casserole is a French word, like a marmite is a specificaly shaped pot for a certain type of stew etc etc etc
Originally a home would have been unlikely to have an oven - bread for example could be baked for you by the village baker in a communal oven (as is still common in some places) - and the old Gaelic concept of a strupach - used to be a tea of scones and tea to drink offered to travellers passing, but as often was a whole meal from whatever was in the cauldron over the fire - shows that the original cookery thing was really just what was in season or avilable was chucked into a pot and stewed to use the heat from the always on open hearth.
As in Scotland - 'you'll have had your tea then' is a well kent saying.....