Trim both ends of the daikon, peel, and cut out 8 big rounds about ½-inch thick. Cut avocado flesh into small cubes. Prepare a pot, Put daikon in the pot and cover with water. Boil, then simmer for about 20 minutes. (You want the daikon to be soft enough to bite, but not mushy.) Drain the daikon and let cool. Prepare a serving plate. Put 4 radish rounds on the dish. Mix crab meat, avocado and lemon juice. Divide the crab mixture into 4 equal portions, place one portion on each of the daikon round. Cover the crab mixture with another daikon round. Now you have 4 burgers. Top the burgers with a little Dijon mustard.