1. Place one cup flour, 1/4 cup cornstarch, baking powder, salt, and chili powder into a large bowl. Add the seltzer and lime juice and mix well. (It will be about the consistency of thin pancake batter.)
2. In another bowl, mix together remaining 1/4 cup flour and 1/4 cup cornstarch.
3. Heat three to four inches (seven and a half to 10 centimeters) of oil in a deep pan. (Don’t skimp on the oil or the fish may stick to the bottom of the pan.)
4. While the oil is heating, cut the fish into one-to-two-inch (two-and-a-half-to-five-centimeter) nuggets and pat dry. Dredge the fish in the flour/cornstarch mixture. Shake off any excess. (This allows the batter to stick to the fish.) Dip dredged fish into batter, allowing any excess to drip off before carefully placing the fish into the hot oil.
5. Cook the fish in batches until golden brown, being careful not to crowd the pan. Drain on paper towels or on a cooling rack set up over paper towels. While still hot, sprinkle very lightly with some additional salt and chili powder.
6. While fish is frying, prepare avocado crème by combining all ingredients in a large bowl. Blend with an immersion blender until smooth and creamy.