Gather the ingredients.
In a large frying pan, melt 6 tablespoons unsalted butter (or vegetable oil) over medium heat and make sure it doesn't brown. Add ½ cup finely chopped onion and fry gently for about 3 minutes or until soft and translucent.
Turn the heat up ever so slightly and add 1 pound mashed potatoes (about 2 cups) and 1 cup leftover vegetables (finely chopped) . Add chopped meat if using. Fry for at least 10 minutes, turning everything over continuously in the melted butter to ensure that the potato and vegetables are thoroughly reheated. You are also aiming to brown (but not burn) the outside edges of the vegetables, so occasionally press the mixture into the pan to brown a little before turning.
Finally, when the mixture is heated right through, give the vegetables one long final press onto the base of the pan with a spatula and leave to cook for 1 minute. Flip over and repeat. Season with salt and freshly ground black pepper .
Serve with either a fried or poached egg on top. Enjoy.
