Preheat the oven to 180°C.
Arrange the peppers in a small roasting tray and drizzle with the olive oil. Season well and roast for 15 minutes until soft. Drain the tuna steak and split into large chunks.
Remove the pepper from the oven and discard any skin that is coming away from the flesh. Arrange slices of tomato across the base of serving plates. Spoon the peppers on top along with any oil from the tray. Season well then sit chunks of the tuna steak on top. Garnish with the spring onion, black olive slices and some basil leaves.
Finish by garnishing with a sprig of tarragon leaves. Serve immediately.