Place the fish in a large shallow bowl and pour the milk over it. Cover and chill for 30 minutes.
Preheat the oven to 200 degrees F. Mix the flour, spice mix, and salt in a pie plate lined with a piece of parchment paper. Working in batches, remove the fillets from the milk and drain off the excess. Transfer to the flour mixture and, pulling up the sides of the parchment, turn to coat on both sides. Flour only as many fillets as will fit in a large skillet at one time.
Heat 2 tablespoons of the oil in a large skillet over high heat until just smoking. Add the fillets and cook until golden brown, about 2 minutes per side. Transfer to a heatproof plate and keep in the oven while you cook the remaining fillets. Repeat the procedure with the remaining fillets, flour mix, and oil. Serve with lemon wedges and Tartar Sauce.
Mix all ingredients together in a small bowl. Store in an airtight container.
Mix all of the ingredients in a bowl. Refrigerate until ready to use.
Recipe courtesy of Sara Moulton, "Sara Moulton Cooks at Home", Broadway Books, 2002