Preheat the air fryer to 400°F (200°C).
Slice the russet potato into 1/2-inch thick wedges, toss with half the avocado oil and a pinch of salt, then air fry for 15-18 minutes until tender and browned.
Pat the cod fillets completely dry with paper towels to ensure the batter adheres properly.
In a small bowl, whisk together the flour, baking powder, garlic powder, salt, and pepper.
Stir in the light beer and egg white until a smooth, slightly thick batter forms.
Dip each cod piece into the batter, allowing the excess to drip off, then lightly mist with the remaining avocado oil.
Place the fish in the air fryer basket in a single layer and cook at 375°F for 10-12 minutes, flipping halfway through, until the exterior is shattering-crisp.
Serve the hot fish alongside the potato wedges with a fresh lemon wedge if desired.