Gather all ingredients and prepare your workspace.
In a large mixing bowl, combine sifted flour, baking powder, salt, and pepper. Gradually whisk in beer or sparkling water until smooth.
Heat oil in a deep frying pan over medium-high heat until shimmering (around 350°F).
Dip each cod fillet into the batter, allowing excess to drip off. Carefully place in hot oil and fry for about 4-5 minutes per side until golden brown.
Repeat the dipping process with onion rings, frying for 2-3 minutes until crispy.
Serve on a platter with your favorite dipping sauces while hot.