Start by whisking together the flour, baking powder, salt, and black pepper in a large bowl.
Gradually add the sparkling water while stirring until you get a smooth batter.
Cut the lemon into wedges and keep them ready for serving.
Heat the oven to 400°F (200°C).
Melt 2 tablespoons of butter in a large skillet over medium-high heat.
Coat each cod fillet in the batter, letting the extra drip off, and carefully place them in the skillet.
Cook the cod for about 3 to 4 minutes on each side until the batter turns crispy and golden.
Set the cooked cod aside once done.
In the same skillet, melt another 2 tablespoons of butter and lightly cook the minced garlic until aromatic.
Add the juice from half the lemon and the chopped parsley, stirring everything together to make the lemon butter sauce.
Arrange the roasted potatoes on a serving dish, lay the crispy cod fillets on top, and pour the lemon butter sauce over the fish.
Garnish with extra fresh parsley and serve alongside the lemon wedges for squeezing.