In a bowl, combine the leftover mashed potatoes, cheese, milk, flour, rosemary, dried onion, paprika, salt and the egg yolk in a bowl. Stir it together well until it becomes smooth and uniform. Use a large ice cream scoop to scoop out uniform portions of the batter and form them into croquettes. You should get 8. Have the lightly beaten whole egg set up in a bowl and the breadcrumbs set up in another bowl.

Dip each croquette in the egg before coating it in the breadcrumbs, then set them on a small platter once they're breaded. Take a large skillet and fill it with about 2 inches of vegetable oil. Heat it up over medium high heat. It will shimmer and start to bubble a little when it's hot enough. Pan fry the croquettes in two batches of four so that the pan doesn't get over crowded. They need to cook for about 2-3 minutes on each side to get golden brown and crispy. Remove them to a plate lined with paper towel to blot them and repeat with the second batch. Once they are all done, serve them immediately and enjoy!