Soak planks in cold water 10 to 20 minutes. Cook prawns 8 to 10 minutes or until they are no longer glossy in appearance.
Cook potatoes in boiling water until tender. Drain, cut in half and set aside.
Peel oranges. Cut segments from membrane with a sharp knife, collecting enough juice to make 3 tablespoons and reserving segments separately. Whisk together orange juice, 1/2 cup oil and vinegar. Season with salt and pepper.
Combine prawns, remaining 2 tablespoons oil, fennel, shallot and lemon zest in a large bowl. Season with salt and pepper.
Arrange prawns under the fennel mixture in a single layer on the planks, dividing prawns and fennel evenly. Grill over medium-high heat until prawns are cooked and plank is well-charred. Just before removing from grill, drizzle each with 1 teaspoon vinaigrette.
Toss lettuce lightly with some of the vinaigrette. Gently mix in blackberries, potatoes, orange segments and additional vinaigrette as needed.
Divide salad onto four plates. Slide prawns and fennel over salads and serve.