My recipe is a wee bit different from rtebalt's.
Bubble and Squeak II
4 tbs butter
cup onion, finely chopped
leftover mashed potato
any leftover vegetables, cabbage, swede, carrots, peas, Brussels Sprouts, finely chopped
salt and freshly ground black pepper
bacon pieces (optional)
In a large frying pan melt the butter, add the chopped onion and fry gently for 3 mins or until soft.

Turn the heat up slightly and add the mashed potato and vegetables. Fry for 10 mins turning over in the melted butter two or three times ensuring the potato and vegetables are thoroughly reheated plus you are aiming to brown the outside edges but not to burn the bubble and squeak.
Press the potato mixture on to the base of the pan with a spatula and leave to cook for 1 min. Flip over and repeat. Serve.