Place the sweet potato into a large pot and cover with water, making sure the sweet potato is completely submerged. Cover with a lid and turn the heat to high. Bring to the boil.
Remove the lid and continue to boil for 15-20 minutes or until sweet potato is cooked through.
Meanwhile, in a medium bowl, whisk olive oil, lemon juice, honey, lemon zest, garlic, and oregano.
Combine vinaigrette with tomatoes and capers. Set aside.
Once sweet potato is cooked, drain the water from the pot. Add the butter beans and cracked black pepper to the pot and mash until smooth. If mixture is dry and difficult to blend add 1 tablespoon reduced fat milk. Keep warm.
Spray a large, non-stick frying pan with olive oil and set over medium-high heat. Add tuna to the pan, cook 2-3 minutes each side or until fish begins to flake when tested with a fork but is still pink in the center.
Remove steaks from the pan and rest for 1-2 minutes before serving to finish the cooking process.
Serve tuna steaks with salsa, mashed sweet potato and a lemon wedge.