Blend all ingredients slowly in a blender until smooth. Chill until ready to serve.
Place sliced red onions in a medium to large Mason jar.
Bring water, sugar, and vinegar to a boil over medium-high heat. Let boil for 2-3 minutes.
Pour hot mixture into the Mason jar over the sliced red onions. Tightly screw on lid and chill for 2 or more hours
Place shredded cabbage in a large bowl.
Separately mix mayo, sugar, salt, lime juice, and vinegar.
Pour mixture over cabbage and mix well. Cover and chill until ready to serve.
Cut cod into 2-4 inch pieces. Set aside.
In a medium-sized bowl mix flour, cornstarch, salt, and baking powder together.
Add club soda and whisk well.
In a large nonstick pan, pour oil in until it comes 3/4-1 inch up the side of the pan.
Heat over medium-high heat until a drop of water in the oil sizzles.
Take a piece of cod and dredge it well in the batter, then using tongs lay slowly into the hot oil. Hold the fish for a minute with the tongs before releasing into the oil so the fish doesn't stick to the bottom. If you hold it for a minute with the tongs and let the bottom fry for 10 seconds, it will harden enough to keep from sticking.
Then release the fish into the oil. Fry several pieces of fish at a time. Cook 3-4 minutes on each side until lightly golden.
Once cooked, remove from oil and let excess oil drip off. Then transfer onto a baking sheet with a wire rack on top of it. Lightly sprinkle some salt on top of fried fish. Let cool slightly before serving in tacos.
To make tacos, layer the slaw on the tortillas, then top with fish, pickled onions, avocados, avocado cream sauce, and if desired, sliced jalapenos, and fresh lime juice.