Season salmon portions with salt and pepper on both sides.
Preheat a large skillet on medium-high heat until it is HOT!
Add Oil and butter to the skillet and swirl around to mix and coat the entire pan.
Add salmon fillets and sear on both sides about 2 minutes per side or until cooked through. DO NOT OVERCOOK or your fillets will be tough and dry.
Remove fillets from the pan to a separate dish. Drain some of the oil from the pan and turn the heat down to medium-low. At this time add the bun halves to the pan cut side down and toast until golden brown.
Assemble your salmon burger by putting about 1 tablespoon of tarter sauce on each half of the bun, following with lettuce, tomato slices and sweet onion. Finally top it off with the seared salmon fillet.
Best when served right away.