Prepare dressing: Whisk all ingredients together in a small bowl and set aside.
Steam or boil potatoes until tender, about 10 minutes. Drain well and set aside to cool.
Preheat BBQ or cast iron grill pan. Lightly coat asparagus and tuna fillets with a little olive oil and sear each side 2-3 minutes until lightly coloured. Take care not to overcook so tuna is rare inside and asparagus is just tender but still bright green.
Remove from pan and set aside for 5 minutes to cool. Cut tuna into diagonal slices and cut asparagus spears in half. To serve, arrange all ingredients on a serving platter. Just before serving drizzle prepared dressing over salad, season with freshly ground black pepper and scatter with fresh parsley.