On a platter, spread the watercress and mixed spring greens over the surface. Scatter the grapefruit, oranges, avocado, onion and raspberries evenly. Place the pine nuts in a heated pan (without oil) and lightly toast, watching and stirring constantly as pine nuts burn easily. Sprinkle the toasted pine nuts on the salad.
In a small bowl, whisk together the grapefruit juice, vinegar and mustard. Slowly whisk in the oil until the mixture is emulsified. Season the dressing with salt and pepper to taste. Drizzle over the salad and serve immediately.