Place the chopped potatoes in a pot of heavily salted water and bring to a boil. Cook until fork-tender. Drain and mash the potatoes, then set aside.
In a skillet, cook the ground beef over medium heat until browned.
Add the diced onion, carrot, celery, and garlic. Sauté for 2–3 minutes.
Sprinkle in the flour, stirring well, and cook for another 1–2 minutes.
Stir in the tomato paste, beef broth, rosemary, and thyme. Cook until the sauce is reduced and thickened.
Remove the rosemary and thyme sprigs. Taste and adjust seasoning with salt or beef bouillon paste for more flavor. Set the beef mixture aside to cool slightly.
Mix the mashed potatoes with the parsley and beef mixture until fully combined.
Scoop portions of the potato mixture and shape into balls. Press a cube of mozzarella cheese into the center of each ball and reshape to enclose the cheese completely.
In a bowl, whisk together the egg, flour, and water until smooth, adjusting the water for a slightly thick batter consistency.
Roll each croquette in the batter, then coat in panko breadcrumbs. Ensure they’re evenly covered.
Heat oil in a deep pot to 350°F (175°C). Fry the croquettes in batches, turning occasionally, until golden brown on all sides. Remove and drain on a paper towel-lined sheet.
For the perfect dipping sauce, combine sour cream with finely chopped chives, a dash of onion powder, and a pinch of salt. For exact measurements and additional tips, visit my Garlic Parmesan Smashed Potatoes recipe on my website. Sprinkle the croquettes with extra parsley and serve warm with the sour cream onion and chive sauce or a squeeze of fresh lemon for added brightness.