Prep the salmon. Preheat an oven to 400°F (205°C), then line a rimmed baking sheet with parchment paper. Pat the salmon filets dry on all sides with paper towels, then season with 1 tsp Kosher salt.
Make the spicy mayo. In a small bowl, whisk together ½ cup Kewpie mayo and 2 tsp Sriracha until well mixed. Reserve half of the sauce for serving. Brush the remaining spicy mayo on top of each salmon filet.
Coat with sesame seeds. Place ½ cup mixed white and black sesame seeds in a shallow bowl, then stir to mix. Firmly press each piece of salmon, mayo-side down, into the seeds. Transfer, seed-side up, to the prepared baking sheet.
Bake the salmon. Transfer to the oven and cook for 12-15 minutes, or until the salmon is fully cooked and reaches an internal temperature of 120-125°F.
While the salmon cooks, make the miso sauce. In a small bowl, combine ¼ cup low-sodium soy sauce, 2 Tbsp sugar, 2 tsp white miso paste, 2 tsp sesame oil, 2 tsp rice vinegar, 2 tsp toasted sesame seeds, and 1 tsp ginger paste. Mix well, then taste and adjust seasoning as needed.
To serve: Place sesame-crusted salmon over rice, then drizzle with additional spicy mayo. Serve with the miso dipping sauce on the side and lots of sliced green onions, cilantro, and lime wedges. Additional topping ideas: pickled ginger, sliced cucumber, and avocado!