Peel avocado and cut in half. Finely mash one half with juice of one lemon, crème fraîche and mayo, season with salt and pepper. Cut the other half of the avocado into wedges and sprinkle with the remaining juice of one lemon.
Spread bun halves with avocado cream.
Place tomatoes, avocado wedges, lettuce, onion and salmon on the lower halves, shape the upper halves and bring to the table.