Whisk together lemon juice and black pepper in small bowl. Add mayonnaise, miso paste, Dijon mustard, and cayenne pepper to lemon-pepper mixture; whisk together.
Spread the lemon-pepper mixture over salmon fillets. Reserve about a tablespoon for later use.
Cover salmon with plastic wrap and refrigerate for 30 minutes.
Preheat oven to 450 degrees F (230 degrees C). Line a baking sheet with parchment paper or silicone baking mat.
Place fillets on the prepared baking sheet. Spread remaining lemon-pepper mixture on fillets without letting it pool around base. Sprinkle with a pinch more black pepper and a generous amount of sea salt.
Bake in the preheated oven until the fish flakes easily with a fork, 10 to 15 minutes.