Step 1Place the 1-inch cauliflower pieces in a 4-quart heavy-bottomed saucepan, cover with the cream and bring to a boil. Reduce the heat to a low simmer, season with one-fourth teaspoon salt and cover the pot. Cook until soft, about 15 minutes, paying attention not to scorch the bottom. Strain the excess liquid and puree the cooked cauliflower in a food processor. Pass through a fine-mesh strainer and adjust seasoning with three-fourths teaspoon salt and one-fourth teaspoon white pepper, or to taste. Set aside.
Step 2In two small bowls, separately toss the watermelon radish and jicama each with one-half teaspoon olive oil, one-fourth teaspoon lime juice, one-fourth teaspoon lime zest and a pinch of salt. Set aside.
Step 3Heat the oven to 325 degrees. For the crumble, toss the brioche with the clarified butter in a bowl. Spread the brioche on a sheet pan lined with parchment paper or Silpat and toast until lightly golden, 5 to 7 minutes. Place the brioche onpaper towels and allow to cool. Place the chopped hazelnuts on a lined sheet pan and toast until lightly golden, about 10 minutes. Cool. Combine the toasted brioche, finely chopped cauliflower, minced parsley, chives and chervil, and the hazelnuts. Season with 1 1/2 teaspoons olive oil, one-eighth teaspoon salt and a tiny pinch of white pepper, or to taste.
Step 4For the mizuna emulsion, first blanch the mizuna in a saucepan of salted boiling water, just long enough to bring out the color. Strain and shock in an ice bath. Strain again, and puree the mizuna in a blender or food processor. Pass the puree through a fine-mesh strainer. Add the vegetable stock to the puree. Season with a pinch of salt. Just before serving, emulsify the puree in a blender or using an immersion blender with 1 tablespoon olive oil to incorporate until light and frothy.
Step 5Place about one-fourth cup cauliflower puree in each of six glasses. Top with 1 1/2 tablespoons each of the dressed watermelon radish and jicama. Cover with 1 1/2 tablespoons of the crumble. Finish with a few tablespoons of froth from the mizuna emulsion. (There will be extra mizuna emulsion.) Serve immediately.