Cut the salmon into large chunks. Put about 1/4 of it into a food processor with the mustard and pulse until the mixture becomes pasty.
Add the shallots and remaining salmon and pulse until the fish is chopped and combined with the puree. You will want there to be chunks of salmon for texture, but no piece should be bigger than a quarter of an inch.
Place the mixture into a bowl and mix in cornichons, breadcrumbs, salt, pepper, and any other seasonings you may want.
Shape the mixture into patties. 2 lbs of salmon should make about 5 patties.
Place the butter or olive oil into a pan and heat to medium/high. Cook the burgers for 3 - 5 minutes per side turning once. You can also place them on a hot grill and cook.
Serve with buns, butter, lettuce, and any condiments you like!