Preheat your air fryer to 400°F to ensure a hot start for the potatoes.
Scrub the Russet potato and slice it into even 1/2-inch wedges, then toss them in a bowl with 0.5 tsp avocado oil and 0.25 tsp sea salt.
Arrange the potato wedges in a single layer in the air fryer basket and cook for 15-18 minutes, shaking the basket halfway through until they are golden and tender.
While the potatoes cook, pat the cod fillets extremely dry with paper towels; removing moisture is the secret to a batter that stays put.
In a shallow bowl, whisk together the all-purpose flour, baking powder, garlic powder, and the remaining salt and pepper.
Pour in the light beer and whisk gently until a smooth, thick batter forms, being careful not to over-mix and lose the carbonation.
Dredge each cod fillet into the batter, ensuring it is fully submerged and coated, then lift and let any excess batter drip back into the bowl.
Lower the air fryer temperature to 375°F, lightly grease the basket with the remaining oil, and air-fry the fish for 10-12 minutes until the coating is crisp and the fish flakes easily.