Dry the fish or onion or vegetables very thoroughly, and coat well with the cornstarch, then set on a large plate or platter and put aside for a few minutes to let dry. Set a wire rack out over a platter lined with several layers of paper towels.
In a medium bowl (flatter works well), stir the almond meal, tapioca starch, gluten free flour, garlic powder, pepper and salt. Add the sparkling water and stir until smooth.
Heat your frying oil slowly on medium or medium high heat in a deep, large saucepan, Dutch oven, or a deep fryer. I like to use a combination of vegetable oil and canola oil. Use enough oil so the saucepan is half full with oil. Use a deep fry or candy or thermometer to check the oil temperature bring to temperature of 350 to 375 degrees. You want to keep it at that temperature, which means when you add fish or vegetables, you may need to adjust the heat on the burner.
When the oil is up to temperature, dip a piece of fish (onion or vegetables) in the batter shake off the excess batter and set carefully in the oil with a tongs or slotted spoon. Repeat for 2 more pieces of fish. Fry three pieces in each batch. Turn after 1 or 2 minutes each side. They should be very nice and golden brown. Remove with tongs to the wire rack and fry the remainder of the fish or onion rings/vegetables in small batches. Sprinkle lightly with salt and pepper.