In a large saute pan, heat 2 tablespoons of the olive oil over medium heat.
Add the ciopollini onions and cook, until lightly browned on all sides and slightly softened 8-10 minutes.
In a small bowl, mix together the balsamic vinegar, sugar, water and a pinch of salt, and add to the onions.
Cook the onions, stirring occasionally until the mixture is reduced down to a syrupy texture and sticks to the onions.
Season the tuna steaks on both sides with salt and pepper.
In a large cast iron skillet, heat 1 tablespoon of the olive oil over medium-high heat. Add the tuna steaks to the hot skillet and sear each side for 1 minute for medium-rare.
Place the tuna steaks in serving plates and spoon the onions on top or around the sides. Sprinkle with the mint and serve.