Grilled Salmon Burgers with Horseradish Cheddar & Pickled Onions

INGREDIENTS:
​1 ½ pounds skinless, boneless salmon
2 teaspoons Dijon mustard
1 Tbsp mayonnaise 
½ cup coarse bread crumbs
3 scallions, sliced white and green parts 
1 tablespoon capers, drained
Salt and black pepper
2 tablespoons butter or olive oil
Salt and pepper 
4 slices horseradish cheddar
4 buns
Pickled Red Onions (see recipe for Jenny's Pickled Onions)
Arugula, for serving
Mayonnaise, for serving 

INSTRUCTIONS:
Cut the salmon into large chunks, and put about a quarter of it into a food processor with the dijon. Pulse until salmon becomes pasty. Add the remaining salmon, and pulse the machine on and off until the fish is chopped and well combined, leaving small chunks of the salmon. Be careful not make the mixture too fine.

Stir in the mayonnaise, bread crumbs, scallions, capers and some salt and pepper. Shape into four burgers.
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Heat oil in a nonstick skillet to medium-high (alternatively you can grill the burgers). Cook each side until golden brown, about 3-4 minutes per side.  Be careful not to overcook!  Serve with arugula, pickled onions, mayonnaise, and horseradish cheddar.