Melt the butter in a medium saucepan over medium heat. Add the water and 2 teaspoons salt and bring to a vigorous boil over high heat. Add the cauliflower florets, return to a boil, and cook until the cauliflower is very tender, 10 to 12 minutes. Remove from the heat and strain the cauliflower; reserve cooking liquid separately. Place the cauliflower into a blender and add 1 1/2 cups of the cooking liquid. Cover the blender tightly (use a kitchen towel to cover lid, pressing down so that it doesn't splatter when the blender is turned on.) Process on very low speed at first, then gradually increase the speed to the highest speed on your blender. Process until completely smooth, light, and airy, usually 2 to 3 minutes.Adjust the thickness by adding more of the cooking liquid if necessary, and add additional salt if necessary. 

While the soup is cooking, place the crabmeat in a small mixing bowl. Stir the oil, lemon juice, and mustard together and season with two pinches of salt. Add the oil mixture to the crabmeat and toss gently to coat the crabmeat, trying to keep from breaking up the lumps too much. 

When you are ready to serve the soup, place about 1/4 of the crabmeat mixture into the center of 4 small soup bowls. Pour the hot soup around the crabmeat. Serve immediately.