In a shallow dish, whisk together the egg, mustard, and salt; set aside. Place the potato flakes in another shallow dish.
Heat oil in a large heave skillet over medium-high heat.
Dip fish fillets in the egg mixture. Dredge the fillets in the potato flakes, making sure to completely coat the fish. For extra chispy, dip into egg and potato flakes again.
Fry fish fillets in oil for 3-4 minutes on each side, or until golden brown.