Preheat your oven to 425°F (220°C) and line a baking sheet with parchment paper.
Cut the russet potato into thin wedges, pat them dry with a paper towel, and season with a pinch of sea salt and black pepper.
Arrange the potato wedges in a single layer on the baking sheet and roast for 20-25 minutes until the edges are golden and crispy.
While the potatoes roast, pat the cod fillet extremely dry with paper towels to ensure the batter sticks properly.
In a small bowl, whisk together the flour, cornstarch, baking powder, garlic powder, paprika, sea salt, and black pepper.
Slowly whisk the cold beer into the dry ingredients until a smooth, thick batter forms; keep the batter cold until ready to use.
Heat the avocado oil in a non-stick skillet over medium-high heat.
Dip the cod fillet into the batter, coating it completely, then carefully place it into the hot skillet.
Sear the fish for 3-4 minutes per side until the coating is deeply golden and the fish flakes easily with a fork.
Serve the crispy fish immediately alongside the hot potato wedges.