Trim the rhubarb stalks to fit inside a 1 litre mason jar. Place the stalks in the jar with ginger and bay leaves.
Pour the vinegar mixture over the rhubarb and seal the jar. Give the jar a shake and let cool on the counter for 2 hours before transferring to the fridge.
Place the rhubarb, dill, garlic, mayo, and lemon juice in a bowl and stir to combine. Cover and chill until ready to serve.
Pat the cod dry with a paper towel and season on all sides with salt. Set aside.In a small shallow bowl, whisk to combine a 1/4 cup of the flour and a 1/4 cup of cornstarch.
Roll the fillets in the flour mixture and set them aside.Whisk to combine the remaining flour and baking powder. Pour in the beer. Dip the cod in the batter and place it to the oil. Fry until golden
Spread a little of the tartar sauce on both the top and bottoms of the buns. Place a piece of fish on top of each bottom bun and top the fish with the shaved cabbage and a few pieces of cucumber.