To make the salsa, cut the top and bottom from the grapefruit to make two flat ends. Use a small, sharp knife to cut downwards and remove the skin, taking off all the white pith. Hold the grapefruit in the palm of your hand and cut on either side of the membrane to remove the segments. Dice the segments.
Dice the avocado flesh the same size as the grapefruit. Gently toss the grapefruit and avocado in a bowl with the onion and parsley.
Brush the swordfish with the olive oil and sprinkle with the rosemary. Season well with freshly ground black pepper. Heat a large non-stick frying pan over high heat and cook the swordfish for 2 minutes on each side, until the flesh is just cooked through.
Transfer the swordfish to four serving plates and serve with the salsa.
Substitute salmon, tuna or ocean trout fillets for the swordfish if unavailable.