Marinate the salmon: Whisk soy sauce, sesame oil, grated ginger, minced garlic, and sriracha. Add the salmon fillet and coat well. Marinate 15–30 minutes in the fridge. Pat off excess marinade before coating with sesame seeds.
Preheat the oven to 220°C (425°F). Place the rack in the middle and line a sheet pan for easy cleanup.
Coat the salmon: Separate the egg, discard the yolk, and beat the egg white until foamy. Dip the marinated salmon in the egg white, let excess drip off, then press all sides into sesame seeds until evenly coated.
Bake the salmon on the prepared sheet pan for 10–12 minutes (or 2–3 minutes longer for thicker fillets) until the center reaches 63°C (145°F) and flakes easily. If seeds brown too fast, move the pan to a lower rack.
Make the spicy mayo: In a small bowl, whisk mayonnaise, lime juice, and the remaining sriracha until smooth. For larger batches, whisk just until creamy; do not overmix.
Serve the salmon on a plate with the spicy mayo on the side. Let the salmon rest for a moment before serving.