Place potatoes in a medium pot and cover with water. Place over medium-high heat and bring to a boil. Boil potatoes until fork tender, 15-20 minutes.
Drain potatoes and pour into a large mixing bowl. Add butter, season with salt and pepper and mash until smooth. Set aside and allow mash to cool.
Pour oil into a medium skillet and place over medium heat. Add onion and sauté for 4 to 5 minutes.
Add garlic, beef, salt, and pepper and crumble using a wooden spoon. Sauté mixture until beef has browned. Stir in peas and remove from heat.
Pour beef mixture over potato mixture, season with salt and pepper and completely mix together.
Lightly wet your hands and take ½ cup of the potato mixture and form into a flat, oval patty, about 1 inch thick.
Dredge patties in a thin layer of flour, followed by the eggs and finally into the panko until patties are all completely encased, shaking off any excess.
Place Patties onto a parchment lined baking sheet and refrigerate for 30 minutes.
Fill a tall pot with 3 inches of oil and preheat to 350˚F.
Fry chilled croquettes in the oil, a few at a time until golden brown, about 3 to 4 minutes. Transfer croquettes onto baking sheet lined with a cooling rack. Lightly season with salt and pepper. Serve.