In a medium bowl with a fork combine well mashed potatoes, beaten egg, parsley, bread crumbs and chopped onion if desired.
Taking approximately a 1/4 cup (more or less) at a time of the potato mixture, form into a round, oval or ball shape, then dredge in flour.
In a medium pan add 2-3 tablespoons of oil, on medium heat, heat until oil is hot (not smoking hot), add croquettes a few at a time turning as they brown.
Place on a plate lined with paper towels. Serve immediately. Enjoy!
The croquettes can be assembled 2 days ahead of time, be sure to cover them well with plastic wrap and store in the fridge until ready to make.Any leftovers should be stored in an airtight container in the fridge. They can be re-heated in a low oven or in the microwave. They will keep for 3-4 days in the fridge.Potato croquettes can be frozen either already cooked or uncooked. Freeze them cooked and cooled in a freezer safe bag or container. When freezing uncooked, freeze them on a cookie sheet until firm then move to a freezer safe container. Let thaw in the fridge, then dredge in the flour and cook. They will keep for up to 1 month in the freezer.They can be baked, bake at 400F (200C) for about 20 minutes.