Cut and separate the king crab legs at their joints. Take each piece with the softer side (white side) of the shell facing up and trim off their shells with a knife to extract the meat. Cut off both ends of the meat to make the pieces all the same.
Heat the oil for deep-frying at a temperature of 355 degrees F. Dust the pieces of crab meat with flour, dip them in the tempura batter and deep-fry in the hot oil for 2 to 3 minutes until the batter is crisp. Drain on paper towels.
Thinly slice the red onion. Chop the jalapeno into thin pieces.
Arrange the fried crab legs on a serving dish and sprinkle with red onion and jalapeno. Make the ama-zu-ponzu sauce by mixing the ingredients together and spoon this around the tempura.
Ama-zu-ponzu sauce can be poured over the crab legs. When you do so, serve the dish quickly so that the crispiness of the tempura batter is not lost.
Do not overmix the tempura batter or it will become heavy. The batter is best if there are a few lumps of flour left visible in the mix.