Whisk together the flour, baking powder, salt, and black pepper in a large bowl. Gradually add sparkling water while stirring until the batter is smooth and well combined.
Cut the lemon into wedges and keep them aside for serving.
Heat your oven to 400°F (200°C). In a large skillet over medium-high heat, melt 2 tablespoons of butter. Dip each cod fillet into the batter, allowing the excess to drip back into the bowl, then carefully lay the fish in the skillet. Fry each side for 3 to 4 minutes until the batter turns crispy and golden. Remove the cooked fillets and set them aside.
In the same skillet, melt the remaining 2 tablespoons of butter and sauté the minced garlic until it smells aromatic. Stir in the juice of half a lemon and the chopped parsley, mixing everything thoroughly to create the lemon butter sauce.
Arrange the roasted potatoes on a serving dish. Place the crispy cod fillets on top and spoon the lemon butter sauce evenly over the fish. Garnish with fresh parsley and serve with lemon wedges on the side for an extra zesty touch.