In a medium skillet, heat 2 tablespoons olive oil over medium heat. Add the onion and cook until softened, about 4 minutes. Add 3/4 cup red wine vinegar, 1/4 cup water and the sugar; season to taste with salt and pepper and simmer over medium heat, stirring occasionally, until the liquid has reduced by half, about 5 minutes. Remove from the heat and let cool. Add the capers and raisins; set aside.
In a large skillet, heat 2 tablespoons olive oil over medium-high heat. Pat the tuna dry with a paper towel. Place the flour on a plate and season with salt and pepper. Coat the tuna on both sides with the flour, shaking off any excess. Cook the tuna in the skillet, turning once, for 4 minutes total for rare, 8 to 10 minutes for well-done.
Meanwhile, in a medium bowl, combine the remaining 2 tablespoons vinegar and 1/2 cup olive oil; season to taste with salt and pepper. Add the mixed greens and toss to coat.
Top each piece of tuna with some of the reserved onion-caper mixture and a generous portion of salad.