I got a bag of frozen cod from costco, and the only way it tastes good is fried, so it works well for this! I used one piece of fish, thawed it out and filet it to make 2 pieces.
Combine the flour, salt, baking powder, and dill. Then add milk and egg. Put the fish in the batter and coat both sides. Let the fish sit in the batter for 15 minutes.
2 tablespoons mayo
2 teaspoons relish
pinch of salt
Now, fry the fish until it is golden brown on both sides, and cooked all the way. Spread the sauce on both sides of a bun, add a slice of cheese and the fish!
We loved this, and maybe even more than the mcd's version. Oh and for the bun I used those flat bread bun things from Costco. I think a regular bun would be better :)