How to Make It
In a medium bowl, combine all of the ingredients. Cover and refrigerate for at least 1 hour.
In a medium bowl, whisk together all of the ingredients. Season with salt and pepper.
In a food processor, pulse the scallions and bell peppers until finely chopped; transfer to a medium bowl. Pulse the salmon in the food processor until finely chopped but with some bigger chunks remaining. Add the salmon to the bowl with the scallions and peppers and fold in the breadcrumbs, salt, pepper and 1/3 cup of the harissa mayo. Using lightly oiled hands, shape the salmon into six 3/4-inch-thick patties. Transfer to a lightly oiled plate and refrigerate for 30 minutes.
Light a grill. Set a large cast-iron skillet on the grill and melt 1 tablespoon of the butter in 2 tablespoons of the olive oil. Add 3 of the salmon burgers to the skillet; cook over moderately high heat, turning once, until golden brown and just cooked through, 4 to 5 minutes. Transfer to a plate and repeat with the remaining butter, olive oil and salmon burgers.
Spread some of the harissa mayo on the buns. Top with the salmon burgers, cucumber relish, lettuce and tomato slices and serve.