Heat the oven to 350°F (176°C).
Season the fish generously with salt and pepper.
Make the tapenade: In a small bowl, combine 2 tablespoons of the olive oil and the olives, lemon zest and juice, garlic, and parsley and mix. Set aside.
Spread 2 tablespoons of the remaining olive oil on a rimmed baking sheet.
Add the squash, season with salt and pepper, and roast until just soft, about 10 minutes. Set aside.
Over medium-high heat, heat the remaining 1 tablespoon oil in a large ovenproof skillet.
Sear the tuna for 1 minute on each side.
Then place the tuna in the skillet in the oven for 3 minutes. Remove and let rest for 5 minutes.
Divide the fish and squash between two dinner plates, top with the tapenade, and serve.