Cut the salmon into 1/4-inch pieces, then transfer about 3/4 of the salmon pieces to a food processor. Add in the mustard, lemon juice, capers, fresh dill, onion powder, and garlic powder; pulse the contents until a paste is formed.
Add the rest of the salmon pieces to the processor and pulse a few times to simply break apart the salmon. You want some salmon chunks in there for texture.
Scoop the salmon mixture into a mixing bowl. Add the scallions, panko breadcrumbs, salt, and pepper, and gently mix it all up with your hands until it's well combined.
Line a baking sheet with parchment paper; divide the salmon mixture into four even parts, then form four patties with your hands. Place each one onto the baking sheet.
Refrigerate the patties for at least 30 minutes. While the salmon patties set in the fridge, you can make the caper mayo (see below).
Heat the grill to medium-high heat; make sure your grill grates are very clean.
Once the grill is hot, grease it well to prevent sticking. Use tongs to dip a wad of paper towels into a little bowl of oil (one with a high smoke point like avocado, canola, or sunflower oil) and glide the oily paper towel over the grates to make them slick and shiny. Do this about 4-5 times to ensure they're slick
Place the salmon patties onto the grill, cover, and cook for 2-3 minutes. Gently flip the burgers, cover, and cook for another 2-3 minutes, or until the patties reach an internal temperature of 140-145º F.
Serve the burgers on nice fresh buns and top them however you please!