In a large skillet, cook bacon until crisp. Drain on paper towels.
Meanwhile, add the mashed potatoes, salmon, egg, 1/4 cup mayonnaise, lemon juice, garlic powder, onion and it's juice, red bell pepper, green onions and 1/3 cup of the panko bread crumbs to a medium bowl. When the bacon has cooled enough to handle, crumble 2 slices into the bowl. Mix with a fork until well-combined.
Divide the salmon-potato mixture into 4 equal portions and form them into patties. Pour the remaining panko breadcrumbs onto a small plate. Coat each side of the salmon-potato patties with the panko bread crumbs.
Heat 2 tablespoons each of butter and vegetable oil in a large saute pan over medium-high heat. Cook the patties for 5 minutes per side, or until a crust forms.
To assemble the sandwiches, place each patty on a bun. Spread 1 tablespoon of mayonnaise on the top bun. Top the patty with one slice of tomato, one lettuce leaf, and one slice of bacon.