Preheat the oven to 425°F and line a sheet pan with parchment paper. Pat the salmon dry, set it on the pan and season both sides with the kosher salt and black pepper. Drizzle with 1 tablespoon of the soy sauce.
Bake the salmon until it flakes easily and reaches 145°F in the thickest part, 12 to 14 minutes. Let it rest 5 minutes, then flake it into large chunks with a fork.
While the salmon bakes, combine the rice and 1 1/2 cups water in a small pot. Bring to a boil, cover, reduce the heat to low and simmer 13 minutes. Remove from the heat and let it steam, covered, 5 minutes, then fluff with a fork.
Stir together the mayonnaise, sriracha, remaining 1 tablespoon soy sauce, sesame oil and rice vinegar.
Divide the rice among 4 bowls and top with the flaked salmon, cucumber and avocado.
Spoon the spicy mayo over the top, sprinkle with sesame seeds and add the nori strips if you like.