Place potato in a saucepan and cover with salted water. Cover, bring to the boil over high heat and cook until potato is tender (8-10 minutes), drain and set aside.
Meanwhile, heat 60ml olive oil in a large frying pan, add onion, olives, capers and anchovy and cook over medium-high heat, stirring occasionally, until onion starts to soften (2-3 minutes). Add potato and cook until onion caramelises (3-5 minutes). Season to taste, add parsley and lemon juice, toss gently to combine and keep warm.
Meanwhile, brush tuna steaks with remaining olive oil. Heat a large frying pan over high heat and sear tuna until cooked (1-2 minutes each side for rare). Serve immediately with potato and lemon cheeks.