Cut the tomato into six wedges, then halve each wedge crosswise.
Peel the potato and cut into 1cm thick half-moons. Soak briefly in water, then drain. Place in a microwave-safe dish, cover loosely with plastic wrap, and microwave at 600W for about 2 minutes. Adjust cooking time as needed.
Spread baby greens on a plate. Arrange tomato, potato, and tuna on top. Scatter sliced black olives and finish with a drizzle of dressing.