Heat the oil in a saucepan, add the pepper and onion, and fry gently over a medium heat for five minutes.
Add the remaining sauce ingredients to the pan and simmer for ten minutes, then season, to taste, with sea salt and freshly ground black pepper.
Meanwhile, for the tuna, place the butter and oil in a frying pan on a high heat. Season the tuna with salt and freshly ground black pepper and place into the pan and fry for 2-4 minutes on each side, until completely cooked through or cooked according to your preference.
To serve, spoon the sauce on a plate and top with the tuna steak.