For the dressing: In a small pot, bring the grapefruit juice to a simmer over medium-low heat. Reduce the juice to 1/4 cup. Set aside to cool.
In a medium bowl, whisk together the reduced grapefruit juice, vinegar, mustard, honey and soy sauce and season with salt and pepper. Slowly drizzle in the olive oil and whisk until emulsified.
For the salad: Combine the radishes with the watercress and fennel in a medium bowl and toss with a small amount of the dressing. Arrange the avocado, grapefruit and orange on a platter, drizzle with the dressing and season with salt and pepper. Sprinkle on the serrano, then top with the dressed radish mixture and serve.