Place cod in a 2-qt. saucepan and cover with 2" cold water; bring to a boil over high heat and cook for 20 minutes. Drain cod, return to saucepan, and repeat process twice more. Transfer cod to a bowl and flake with a fork into small chunks; set aside. Heat 1 tbsp. oil in a 12" skillet over medium-high heat. Add garlic and onion; cook until soft, about 5 minutes. Transfer to a bowl; set aside. Whisk flour, salt, egg, and 1 1⁄2 cups water in a bowl. Stir cod, garlic and onion, butter, and cilantro together to form a thin batter.

Wipe skillet clean and add 1⁄2 cup oil; heat over high heat. Working in batches and adding remaining oil as necessary, pour about 1 tbsp. batter into skillet; cook, flipping once, until golden brown, 1–2 minutes. Using a spatula, transfer chips to paper towels to drain. Season with salt and serve hot.