Heat a frying pan and add the spring onions, the butter and a good drop of olive oil to stop the butter from burning.
Meanwhile, remove the black seeds from the cardamom pods using the back of a knife. Place in a mortar (or alternatively use a plastic bag and a rolling pin), add the coriander seeds and cumin seeds and work the pestle or the rolling pin until they are coarsely ground. Don't grind them to a complete powder - you want texture and nuttiness from the spices.
Add the spices to the spring onions and cook, stirring occasionally, until they turn a pale golden-brown but before they start to burn and crisp up. Add the onion mixture to some leftover mashed potato in a bowl and form into four potato cakes using your hands. Place the cakes back into the frying pan and cook until crisp and brown on each side, literally a few minutes, that's all, and remove from the pan.
For a quick sauce, add the cream to the pan used to cook the bubble and squeak cakes with some fresh coriander. Bring to the boil and season well. Serve the bubble and squeak cakes straightaway with the sauce. These are also wonderful served with grilled bacon rashers or gammon steaks.