Preheat your air fryer to 400°F (200°C).
Slice the Russet potato into thin, even wedges and pat them dry with a paper towel.
Toss the potato wedges with avocado oil and a pinch of sea salt, then place them in the air fryer basket in a single layer.
Air fry the potatoes for 15 minutes, shaking the basket halfway through.
While potatoes cook, pat the cod fillets dry and season both sides with garlic powder, paprika, and black pepper.
In a small bowl, whisk together the arrowroot starch and light beer until a smooth, thin batter forms.
Dip each cod fillet into the batter to coat thoroughly, letting any excess drip off.
Move the potatoes to one side of the air fryer basket and place the battered fish on the other side.
Air fry everything together for an additional 10-12 minutes until the fish coating is crisp and the potatoes are deeply golden.
Serve immediately with a fresh lemon wedge and the remaining sea salt.