How to Make It
Place the avocado, lemon juice, butter, oregano, and ¾ teaspoon salt in a food processor. Pulse until nearly smooth, about 30 seconds. Set aside.
Place a medium nonstick skillet over medium heat and coat with vegetable cooking spray.
Sprinkle the salmon with the remaining ½ teaspoon salt and cook until opaque throughout, about 3 minutes per side. Transfer to a plate.
Spread half of the avocado butter on two slices of bread and top with half of the salmon and arugula and two slices of bread. Repeat with the remaining ingredients and top with the remaining bread to make two double-decker sandwiches.