Fry the onions in half the butter until soft, but not coloured. Leave to cool.
Combine the onion with the potatoes, sprouts, cabbage and garlic. Season well. Divide the mix into four and shape into squares or rounds. Dust with a little flour and fry in the remaining butter for about 5 minutes on each side until golden.
For the poached egg
Carefully break your egg into a pan of simmering water. Simmer for about 1 minute, then remove from the heat and allow the egg to sit in the hot water for 10 seconds. Remove the egg with a draining spoon and place on some kitchen paper to absorb any excess water. Serve immediately.