First make the vinaigrette. In a small pan over high heat, bring the blood-orange juice to a boil, and reduce by half. Remove from the heat, and allow to cool.
In a small mixing bowl, combine the reduced orange juice, mustard, vinegar and salt to taste. Whisk to blend, and slowly whisk in 1/2 cup of the olive oil until emulsified. Set aside. Grate and reserve the zest from the orange. Cut any remaining peel and pith from the orange. Over a bowl, cut the orange into segments, reserving the segments and juices released into the bowl.
In a 10-to-12-inch saute pan over high heat, heat 1 tablespoon olive oil, and add the scallions. Saute until the scallions are softened, 30 to 45 seconds. Add the mushrooms, and season with salt and pepper to taste. Saute until the mushrooms are softened, 2 to 3 minutes. Add the radicchio, orange segments and any juice. Toss to wilt, 30 to 45 seconds. Transfer to a platter, and keep warm.
In a small bowl, combine the orange zest, pine nuts, and fennel then set aside. In a 12-inch saute pan over high heat, heat the remaining 2 tablespoons olive oil until smoking. Season the tuna steaks with salt and pepper to taste, and add to the pan. Saute for 4 to 5 minutes, then turn the steaks and saute for another 2 to 3 minutes, for medium rare. An additional 2 to 3 minutes for well done. Transfer to the serving platter, laying the steaks on top of the radicchio mixture. Sprinkle the blood orange vinaigrette over the steaks, and top with the orange zest mixture. Serve immediately.