Cut the salmon into large chunks, and put about a quarter of it into the container of a food processor, along with the mustard. Turn the machine on, and let it run — stopping to scrape down the sides if necessary — until the mixture becomes pasty.

Add the shallots and the remaining salmon, and pulse the machine on and off until the fish is chopped and well combined with the puree. No piece should be larger than a ¼ inch or so; be careful not make the mixture too fine.

Scrape the mixture into a bowl, and by hand, stir in the bread crumbs, capers and some salt and pepper. Shape into four burgers. (You can cover and refrigerate the burgers for a few hours at this point.)

Place the butter or oil in a 12-inch nonstick skillet, and turn the heat to medium-high. When the butter foam subsides or the oil is hot, cook the burgers for 2 to 3 minutes a side, turning once. To check for doneness, make a small cut and peek inside. Be careful not to overcook. Serve on a bed of greens or on buns or by themselves. Condiment as you like.