Crab sandwiches are delicate, so we prefer thin brioche toast with just crab, soft lettuce, a bit of creme fraiche, and some avocado.
Mix lemon juice, salt, creme fraiche, shallot, and chives. Season with pepper. Fold in crab. Cover, and refrigerate for 30 minutes. Divide lettuce, avocado, and crab among 4 bread slices. Sandwich with remaining slices.