Blizt bread, herbs, onion and zest, but not too finely, chunky is best, stir in onion and salt and pepper. The consistency should be that it is barely holding together; if it is too dry add some water a drop at a time.
Heat oil in oven proof pan over high heat, sear salmon, skin side down for 2-3 minutes, remove pan from heat. Push pieces together and spread top with mustard and then the mayo. Place bread mix on top, patting down. Scatter butter pieces over top. Place in oven for 10-12 minutes or until top is nicely brown and crispy.