Place the salmon chunks in the bowl of a food processor. Pulse until finely chopped, scraping down the sides as necessary. Transfer to a large mixing bowl. If you don’t have a food processor, finely chop the salmon by hand.
To the salmon, add the grated onion, breadcrumbs, cilantro, egg white, garlic, salt, pepper, paprika and oregano. Stir well to combine.
Divide the salmon mixture into 4 equal portions. Roll one portion into a ball with the palms of your hands, then flatten into a ½-inch to ¾-inch patty. Repeat with the remaining portions.
Preheat the grill to medium-high heat. Brush the grates with oil (I use avocado oil).
Cook the burgers for 4 minutes (the cooked side should be golden brown and release easily from the grates), then flip and cook for additional 1 to 2 minutes. Take care not to overcook. (See Note below regarding internal temperature.)
Toast the buns on the upper rack of the grill for 1 to 2 minutes.
Spread 1 tablespoon of avocado sauce on the bottom half of one bun and another 1 tablespoon on the top half of the bun. Top with a salmon patty and tomato slices. Repeat with the remaining buns, avocado sauce, salmon patties and tomato. Serve.