With the fries done, increase the heat of the oil to medium-high, add remaining quart of oil if necessary and heat oil to 375 degrees. Meanwhile, thoroughly dry fish with paper towels. Dredge each piece of fish in the flour mixture shaking off excess flour. Dip 1 piece of fish in batter at a time and let excess run off, shaking gently. Place battered fish back into the dish with the flour mixture and turn to coat both sides. (This is messy, but worth it!)
If you are working with a partner in the kitchen you can now add each piece directly to hot oil if or, if you choose, keep pieces in single layer on baking sheet and repeat until all pieces are coated. (It’s best if you have one person frying and another dredging. However, you can do certainly do it in batches if working alone, just be careful not to let it sit too long to avoid the batter sticking to the sheet pan)
Tip: As you add the fish, you will ikely need to adjust the heat to keep it maintained at 375-385. Not adjusting your temperature to maintain oil temperature when frying is a common, and costly, mistake. The fish will not crisp up. But make it too high, and everything will burn. Use an oil safe thermometer for best results.
Fry the fish, stirring occasionally, until golden brown, about 5 minutes depending on thickness. Use a kitchen spider to remove fish from hot oil and transfer fish to a sheet pan lined with a gridded cooling rack.
Tip: If you are unsure about the doneness, remove one piece of fish and use an instand read thermometer to test for a minimum of 145 degrees F.