Make the mustard crust: In a bowl, combine the mustard, lemon juice, fresh herbs, salt and pepper, and stir together. Stir in the olive oil, and then the bread.
Preheat the oven to 450 degrees F. Put the salmon, skin side down, on a parchment-lined baking sheet. Drizzle with olive oil. Spoon the bread mixture onto the salmon and smooth with a spatula to cover completely. Put the baking sheet into the oven and roast until the salmon is just cooked through but still translucent in the center, about 10 minutes.
When the salmon is cooked, take it out of the oven and set aside. Combine the mayonnaise, the chopped arugula, and the lemon juice in a blender and puree. Season with salt and pepper.
To serve, transfer the salmon to a serving platter by holding 2 opposing corners of the parchment paper and lifting the whole thing off the baking sheet and onto the platter, parchment and all. Spoon the mayonnaise into a serving bowl and garnish with remaining arugula. If serving with cocktail bread set the salmon out with the arugula mayonnaise, and the arugula leaves and let guests make open-faced sandwiches by spreading the toast with the mayonnaise, adding a leaf or 2 of arugula, and finishing with a spoonful of the salmon.