Place a rack in the center of your oven and preheat the oven to 400 degrees F. Line a large rimmed baking sheet with foil and generously coat with nonstick spray. Set aside.
Add the buttermilk and 1 teaspoon of the OLD BAY to a large ziptop bag and mix to combine. Add the fish and tightly seal. Refrigerate for 20 minutes.
Meanwhile, cut the potatoes in half lengthwise. Then, cut each half into thirds, also lengthwise, so that you have 6 wedges total per potato. Place the potato wedges in a large bowl. Top with olive oil, garlic, and 2 teaspoons OLD BAY. Toss to coat, then spread in an even layer on the prepared baking sheet, discarding any liquid that collects at the bottom of the bowl.
Bake for 25 minutes, turning once halfway through. Remove from the oven and push the potatoes to the outside edges of the baking sheet.
Place the flour in a large, shallow dish (a pie dish works well). In a second dish, briskly whisk the egg whites and 1 teaspoon of OLD BAY until foamy. In a third dish, stir together the Panko and the last teaspoon of OLD BAY. Remove the fish from the buttermilk marinade, shaking off any excess, then dip in the flour, coating all sides. Shake off the excess flour, then dip in the egg whites to moisten, and finally dip in the Panko. Press the Panko onto the fish’s surface so that it sticks to all sides, then arrange the fish in the empty space on the baking sheet. Repeat with the remaining fillets.
Bake the potatoes and fish together for another 15 minutes, until the fish is golden brown and flakes easily with a fork. Serve with lemon wedges and tartar sauce.
*No buttermilk? Mix 1/4 cup regular milk with 2 teaspoons lemon juice or white vinegar. Let sit 5 minutes (the milk will curdle) then use as directed.
**Do not use a flaky white fish, such as tilapia or snapper, as they will not hold together.
TO MAKE AHEAD: The fish can be marinated for up to 8 hours in the refrigerator prior to preparing.
TO STORE: This recipe is best enjoyed the day it is made. Store leftovers in the fridge for up to 1 day.
TO REHEAT: Let the fish come to room temperature, then place it uncovered on a baking sheet in a 350 degree F oven for a few minutes, just until warmed through.