In a small bowl, whisk together the lemon juice, vinegar, salt and pepper to taste. Drizzle in the olive oil while whisking until it thickens.
Cut out the core of the radicchio and discard, then cut the heads lengthwise in thin slices. Discard any hard parts and toss in a serving bowl with the vinaigrette. Add the rocket, watercress and parsley then mix. Set aside.
In a non-stick frying pan over high heat, heat up 1/2 teaspoon salt. Add the duck breast, skin side down, reduce heat to medium high and cook until crisp and golden brown, about 7 minutes. Turn and cook the other side until browned, about 5 minutes. Cover the pan and let cook until medium rare, about 5 more minutes. Drain on paper towels.
Slice the duck breast into 5mm slices. You can discard the skin if you want it to be less fatty. Add the slices to the salad mixture and sprinkle with pepper. Serve warm.