Start with the dressing. Set a small sauté pan over medium heat. Add the coriander and cumin and toast, shaking once or twice, for about 45 seconds, just until spices have darkened just a shade or two and smell fragrant. Using a mortar and pestle or spice grinder, grind toasted seeds to a fine powder.
In a small mixing bowl, use a fork to combine the garlic, ginger, and grapefruit zest and juice, 1 1/2 teaspoons of the ground coriander and cumin mixture (reserve any extra for garnishing later on), and a pinch of chile powder. Use the fork to slowly beat in the yogurt, followed by the olive oil. Stir in minced herbs, and add sea salt and pepper to taste.
To prepare the salad, cut away any green fennel fronds and reserve for garnish. Cut the core out of the fennel and shave thinly on a mandoline. Trim the celery and shave at a diagonal into thin slices. Shave the shallot. Set shaved vegetables in a large bowl and sprinkle with a pinch or two of sea salt. Toss with a tablespoon of the yogurt dressing and set aside while you prepare the fruit.
Cut off tops and bottoms of the citrus, cut off the peel and pith with a downward motion, and supreme the fruit.
Fold the herbs into the shaved vegetables, and add several more tablespoons dressing. Adjust salt levels as needed. Arrange on a serving platter and tuck citrus wedges throughout. Garnish with any reserved fennel fronds, dill, and cilantro, plus a pinch each of the toasted spice mixture and black pepper.
To best capture the fragrance and flavor, this salad is best enjoyed right away. If making ahead, you'll want to keep all the main elements separate until just before serving.