In a small sauce pot, combine the rice and 1½ cups [2½ cups] water. Bring to a boil, reduce to a simmer, cover, and cook until the rice is tender and the liquid is absorbed, 20 to 25 minutes. Fluff with a fork, cover, and keep warm. Meanwhile, prepare the rest of the meal.

Tuna:

• Pat the fish dry with a paper towel; season lightly with salt and pepper. 

In a large frying pan over medium-high heat, warm 1 to 2 tablespoons oil until hot but not smoking. Add the fish and cook, turning once, until lightly browned, about 30 seconds per side for rare or about 1 minute per side for medium. Transfer to a cutting board to cool slightly. Cut the fish into ½-inch pieces. 

Salmon: 

• Pat the fish dry with a paper towel; season lightly with salt and pepper.

In a large frying pan over medium-high heat, warm 1 to 2 tablespoons oil until hot but not smoking. Add the fish and cook until lightly browned on the bottom side, 4 to 5 minutes. Turn and cook until the flesh is opaque and flaky, 2 to 4 minutes. Transfer to a cutting board to cool slightly. Cut the fish into ½-inch pieces.

Scallops: 

• Rinse and drain the scallops. Pat dry on a paper-towel-lined plate. Season lightly with salt.

In a large frying pan over high heat, warm 1 to 2 tablespoons oil until hot but not smoking. Add the scallops and cook without turning until golden brown on one side and starting to turn opaque, 2 to 4 minutes. Turn and continue cooking for about 30 seconds for translucent or 1 to 2 minutes for opaque. Transfer to a plate.

While the seafood is cooking, start preparing the remaining ingredients.

• Peel the cucumber if desired; cut the cucumber in half lengthwise, then crosswise into ¼-inch-thick half-moons.
• Juice half the lemon; cut the remaining half into wedges for garnish. [Juice 1 lemon; cut the remaining lemon into wedges.]
• Trim the root ends from the scallions; thinly slice the scallions on the diagonal, keeping the white and green parts separate. Set aside the green parts for garnish.  

In a medium bowl, combine the cucumber with as much shichimi togarashi as you like and season generously to taste with salt. Let stand, stirring occasionally, for 10 to 12 minutes. 

In another medium bowl, combine the seafood, lemon juice, white parts of the scallions, and half the sesame-ginger marinade (set aside the remaining marinade for serving). Season generously to taste with salt and pepper. Let stand, stirring occasionally, until the flavors combine, 10 to 12 minutes. 

Transfer the rice to individual bowls and top with the cucumber, seafood, carrots, and nori strips. Garnish the seafood with the sesame seeds and green parts of the scallions. Serve the lemon wedges and remaining marinade alongside.