1. Cook the Beef: Heat a large skillet over medium-high heat. Add the ground beef and cook, breaking it apart with a spoon, until it browns completely. Drain any excess grease thoroughly. Transfer the cooked beef to a large mixing bowl.

2. Combine Ingredients: Add the mashed potatoes, minced garlic, red pepper flakes, chopped fresh parsley, salt, pepper, and optional onion powder to the bowl with the cooked beef. Use your hands or a sturdy spoon to mix everything together until well combined.

3. Form the Croquettes: Take about two tablespoons of the beef mixture and roll it into an oval or cylindrical shape, about 2-3 inches long. Repeat with the remaining mixture, placing the formed croquettes on a plate or baking sheet.

4. Set Up Breading Station: Prepare three shallow dishes. Place the all-purpose flour in the first dish. In the second dish, whisk the large egg until well beaten. Put the 1 cup of Panko breadcrumbs in the third dish.

5. Bread the Croquettes: Working one at a time, dredge each croquette first in the flour, shaking off any excess. Next, dip it into the beaten egg, ensuring it gets fully coated. Finally, roll the croquette in the Panko breadcrumbs, pressing gently to make sure the crumbs adhere well to all surfaces. Place the breaded croquettes back on the plate or baking sheet.

6. Chill (Optional but Recommended): For best results, refrigerate the breaded croquettes for at least 30 minutes.

7. Heat the Oil: Pour about 3-4 cups of vegetable or canola oil into a deep, heavy-bottomed pot or Dutch oven. Heat the oil over medium-high heat until it reaches 350-375°F (175-190°C).

8. Fry the Croquettes: Carefully place 3-4 croquettes into the hot oil, ensuring you do not overcrowd the pot. Fry for 3-5 minutes, turning occasionally, until they turn deep golden brown and appear crispy on all sides.

9. Drain and Serve: Use a slotted spoon to remove the cooked croquettes from the oil. Place them on a plate lined with paper towels to drain any excess oil. Repeat with the remaining croquettes. Serve warm.