Slice the carrot and onion very thinly and soak in water. Slice the lettuce and radish. Cut the lemon in half into semicircles.
Sear each side of the salmon quickly in a frying pan, then plunge into ice water to cool. Pat dry with paper towels.
Diagonally slice the salmon like cutting sashimi. Arrange the lettuce on a plate. Next, place the salmon and lemon, alternating them in a circle on the lettuce. Garish with carrots and onions and drizzle on dressing.