Whisk the lemon juice, olive oil, salt and pepper in a large bowl to make a dressing.
Peel and core the avocado and cut the flesh in to chunks, dropping them directly into the dressing.
Trim any white pith from the grapefruit. Cut into small segments and add to the dressing along with any extra juices. Toss lightly and chill for 10 minutes or so.
Heat the remaining oil in a heavy pan and cook the salmon, skin – side down, until the skin is crisp and the flesh turns pale pink. Turn and cook on the other side, leaving the salmon soft and pink in the middle.
Place the salmon, skin side up on serving plates and top with a big spoonful of avocado and grapefruit.