Heat the oven to gas 6, 200°C, 400°F. Mix all the chopped vegetables together with the olive oil and place in a nonstick roasting pan. Cook for 30 minutes, turning from time to time to ensure even cooking.
Heat a griddle pan and, when hot, cook the tuna on each side: 2 minutes, rare; 3 minutes, medium; and 5 minutes, well done.
Serve the tuna steaks with the roasted vegetables, rocket, seasoning, lemon wedges.