Preheat your oven to 425°F and line a large baking sheet with parchment paper.
Slice the Russet potato into thick wedges and toss them in a bowl with the sea salt and black pepper.
Arrange the potato wedges on the baking sheet in a single layer and roast for 25 minutes until the edges are crispy.
In a medium mixing bowl, whisk together the cassava flour, garlic powder, and paprika.
Slowly pour in the light beer and whisk in the egg white until a smooth, bubbly batter forms.
Pat the cod fillets dry with paper towels to ensure the batter sticks, then dip each piece into the batter to coat thoroughly.
Heat the avocado oil in a large non-stick skillet over medium-high heat.
Carefully place the battered cod in the skillet and sear for 3 to 4 minutes per side until the crust is golden brown and the fish is opaque.
Serve the hot fish immediately alongside the roasted potato wedges with a fresh lemon wedge for squeezing.