Slice the russet potato into thin wedges and soak them in a bowl of cold water for 10 minutes to remove excess starch.
Drain and pat the potato wedges completely dry with a clean kitchen towel.
Toss the potatoes with half of the avocado oil and a pinch of sea salt, then place in the air fryer at 400°F for 15-20 minutes until golden and crisp.
In a shallow bowl, whisk together the arrowroot powder, garlic powder, paprika, black pepper, and remaining sea salt.
Add the egg white and light beer to the dry mixture, whisking until a smooth and slightly frothy batter forms.
Pat the cod fillets dry with paper towels, dip each piece into the batter to coat thoroughly, and let any excess drip off.
Brush the remaining avocado oil onto the air fryer basket, place the fish inside, and cook at 375°F for 10-12 minutes, flipping halfway through, until the exterior is browned and the fish flakes easily.