Lightly butter the slices of bread. Make them into sandwiches with 1-2 slices of cheese in each sandwich, adding any of the optional extras you like.
Heat the oil and knob of butter in a frying pan, over a moderate heat. Put the sandwiches into the hot fat, and fry for a few minutes on each side, until the bread is golden and crispy, and the cheese is beginning to melt.
Remove from the pan, drain on a piece of kitchen paper if they seem a bit greasy. Eat at once while hot.