The combination of fresh seafood with fennel, capers, and tomatoes is typical of Sicilian dishes.
Place rice and 2 1/4 cups water in a medium saucepan, and bring the liquid to a boil over medium-high heat. Cover, reduce to a simmer, and cook until rice is tender and all of the liquid has been absorbed, about 10 minutes. Remove the pan from heat. Using a fork, separate the grains of rice, and transfer rice to a large bowl. Grate 1 tablespoon zest from the lemons, and add the zest to rice. Set the rice aside.
Squeeze the lemons, yielding 1/3 cup strained juice. Place 12 tomato halves, 1 tablespoon chopped parsley, lemon juice, 1/2 teaspoon salt, and 1/4 teaspoon black pepper in a blender; blend until liquefied. With blender running, add olive oil in a slow, steady stream to the tomato dressing.
Add yellow bell pepper, onion, fennel, the remaining tomato halves, capers, the remaining 5 tablespoons parsley, marjoram, and tomato dressing to rice; toss to combine.
Coat a saute pan with cooking spray, and set over medium-high heat. Cut the tuna steaks into 1-inch-wide strips, and season with the remaining 1/4 teaspoon salt and 1/4 teaspoon black pepper. Place the tuna strips in hot pan, and sear on all sides until browned and medium rare, about 1 minute per side. Remove the pan from heat, and cut each strip into 1/2-inch-thick slices. Divide the rice salad and the tuna slices among six bowls, and serve immediately.