Prepare the balsamic caramel by combining the balsamic vinegar and caster sugar together in a saucepan. Bring to a simmer over a moderate heat and continue to simmer until it has reduced by half and thickened.
Preheat the oven to 200° C.
Arrange the peppers and spring onions in a roasting tray. Season and drizzle over half of the extra-virgin olive oil. Roast for 10-15 minutes until soft. Remove from the oven and pour over the rest of the olive oil. Cover and leave to cool to room temperature.
Rub the tuna steaks with the groundnut oil and season with salt and pepper. Heat a cast iron frying pan over a high heat until hot. Sear the tuna steaks for 1-2 minutes on all sides, then transfer to the oven to finish cooking for 4-6 minutes.
Transfer the peppers and spring onion to the oven at the same time as the tuna steaks go in to reheat. Reheat the balsamic caramel at the same time over a low heat. Spoon the peppers and spring onions onto serving plates. Top with the tuna steaks and spoon the balsamic caramel on top.