Cut three-quarters of the salmon into 1/4-inch pieces. Put in a large bowl. Cut the rest of the salmon into chunks; transfer the chunks to a food processor along with the mustard, mayonnaise, lemon juice, lemon zest and cayenne. Pulse to make a paste. Add the pureed salmon mixture to the bowl with the diced salmon. Add the scallions, 2 tablespoons panko, 1/2 teaspoon salt, and black pepper to taste. Gently mix until just combined.