First make your relish. Bring vinegar, mustard seeds, and coriander seeds to a boil in a medium non-reactive saucepan. Cook until reduced by half and slightly syrupy. Remove from the heat, add the remaining ingredients, and gently toss to coat. Season with salt and pepper, to taste. Cover and refrigerate for at least 1 hour before serving.
For fried chicken heat vegetable oil in a large saucepan to 375 degrees F (190 degrees C). Season chicken with salt and pepper, coat with flour then with beaten egg and at the end toss it in Panko. Place it in the hot oil and fry until golden brown.
Now assemble your sandwich. Put 1 tbsp mayo on the bottom, place the arugula on mayo, then your delicious fried chicken and top it generously with pickle relish.