1. Heat oil in a nonstick pan over high heat. Pat fish dry. Cook fish until golden brown on outside but still quite rare in center, 1 1/2 to 2 minutes per side. Remove from pan; let cool slightly. Meanwhile, toast nori sheets, 1 at a time, using tongs, over a medium-high flame until they begin to shrivel and are fragrant.

2. Mix together lemon juice, soy sauce, and Sriracha. Slice avocado. Divide rice among 4 plates, and drizzle with half the sauce. Arrange avocado and cucumber on top of rice. Break tuna into chunks, and divide among plates. Drizzle with remaining sauce, and top with cilantro. Break nori into pieces, and serve on the side.