Heat frying pan over medium head. Lightly brush the tuna fillets with olive oil and cook 2-3 minutes on each side, until done to your liking. Remove from the pan and leave to cool, then slice or break into large chunks.
Meanwhile, cut the top and bottom off each grapefruit, and stand them on one cut end on a chopping board. Remove all skin and pith by cutting down around each grapefruit, to the chopping board. Segment the grapefruit by cutting out the flesh in wedges from between the membranes.
To make the dressing, whisk the vinegar and oil in a bowl, and season with salt and freshly ground black pepper.
Place the tuna, grapefruit segments, avocados, rocket and dressing in a large salad bowl. Gently toss together to combine, then serve immediately.
Variation: Instead of fresh tuna, use smoked trout, or canned tuna in a flavoured oil, and omit step 1. You can also replace the pink grapefruit with the sweet ruby red grapefruit, orange segments or diced fresh mango; or omit the grapefruit and add tomato wedges, black olives and fresh mint leave.
Per serving: 408 calories, 34 g protein, 28 g fat (7 g saturated fat), 4 g carbohydrate (4 g sugars), 2 g fibre, 66 mg sodium