1. Sauté onions in butter in a medium frying pan over high heat until softened, 1 minute. Transfer to a large bowl. Add mustard, garlic powder, salt, crab, 1 egg, and 1/4 cup panko and stir until blended.
2. Form mixture into 8 balls and set them on an oiled baking sheet. Whisk remaining egg with 2 tbsp. water on a small plate. Put remaining 1/2 cup panko on another plate. Dip balls in egg, then roll in panko.
3. Meanwhile, pour oil into a medium pot, insert a deep-fry thermometer, and heat over high heat to 350°. Cook 3 or 4 balls in oil at a time, turning often, until medium brown, 4 minutes. Transfer croquettes to paper towels. Serve with hot sauce.