There were several varieties, tomato, oxtail, pea, mock turtle (dont ask)

The ration was developed jontly by Heinz and I.C.I. (UK - Imperial Chemicals Industries)
There was a hollow central well, with a very rapid heating element. You popped the top cover, as shown, and put a cigarette end to the element. Took about 30 seconds to boil the can.
These were hugely popular amongst UK and Canadian troops, especially in the winter of 1944-5 when they were first issued in mass. Very expensive to produce though.