Chop the pineapple into small pieces. Put in a bowl with the pepper, chilli and coriander. Add 1 tablespoon of the reserved pineapple juice (discard the rest) along with the lime juice and stir well to combine. Set aside.
Heat a griddle pan to high. Brush the tuna with the oil and season with freshly ground black pepper. Griddle for 1½ mins each side for medium or 3 minutes each side for well-done. You may need to do this in 2 batches. Serve the seared tuna steak with the salsa and the watercress.
Cook’s tip: you can make the salsa up to 2 days ahead and keep it covered in the fridge.