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
• black pepper
• 2 tablespoons butter or olive oil
• pepper
• 4 slices horseradish cheddar
• 4 buns
• Pickled Red Onions (see recipe for Jenny's Pickled Onions)
• Arugula, for serving
• Mayonnaise, for serving