Peel the oranges with a sharp knife, removing all the pith. Cut into fine slices.
Drain and flake the tuna. Drain and separate the anchovies, reserving the oil for the dressing.
Heat the olive oil gently in a frying pan. Add the red pepper and fry for about ten minutes until the slices soften and caramalise a little. Be careful not to overheat the oil.
Put the orange slices into a bowl and toss with the contents of the frying pan, chopped spring onions, garlic and vinegar.
Finish with the flaked tuna, the anchovies and their oil and the olives. Leave to marinate for 1-2 hours.