Heat 2 tablespoons of olive oil in a large skillet over medium heat, then sear the chicken legs on all sides until lightly golden.
Pour in the white grape juice, let it simmer for a minute, then add the chicken broth, rosemary, and garlic, cover the skillet, reduce heat to low, and cook for 45 minutes until the chicken is tender.
While the chicken cooks, rub the potatoes with the remaining olive oil, wrap them in foil, and bake at 425°F (200°C) for 1 hour until soft.
Let the baked potatoes cool slightly, then peel and shred them into a large bowl.
Remove the cooked chicken from the skillet, take out the bones and set them aside, shred the meat finely using a mixer or your hands, and add it to the bowl with the potatoes.
Mix in half the mozzarella, season with salt and pepper, and stir everything together until well combined, then let it cool completely.
Divide the mixture into six portions, flatten one in your palm, add a pinch of mozzarella and a clean bone to the center, then shape it into an oval croquette with the bone sticking out; repeat with the rest.
Place the shaped croquettes in the fridge for about 1 hour so they firm up and hold their shape when frying.
In one bowl, mix flour, cayenne pepper, mustard powder, and a bit of salt and pepper; in a second bowl, beat the eggs; in a third, pour in the panko.
Dip each croquette in the flour, then egg, then panko, then back into the egg and panko for an extra crispy coating, and fry in hot oil at 350°F (180°C) for about 10 minutes until golden brown and crispy.