One day ahead, make the labne by placing the yogurt in a cheesecloth-lined sieve, over a large bowl. Refrigerate overnight to drain the liquid.
For the dressing, combine the vinegar, orange juice, shallot, sea salt and pepper in a small bowl and let sit for 10 minutes. Whisking continuously, slowly add the olive oil to the vinegar mixture. Taste and adjust seasoning, as necessary, then set aside.
To assemble the salad, combine half of the dressing with the fennel, radish and half of the herbs on a large serving platter. Remove the labne from the cheesecloth and discard the liquid. Arrange the orange slices on the fennel and place the labne in a rough half-dome in the center of the dish. Drizzle with the remaining dressing. To finish, tear the remaining herbs and fennel fronds over the salad and scatter with hazelnuts and a sprinkle of fleur de sel.