Peel the grapefruit with a small paring knife. Working over a bowl to catch the juice, separate the grapefruit segments from the membranes; reserve any juice that collects in the bowl. Halve the grapefruit sections crossways and transfer to a serving bowl. Add the capsicum, celery, onion, parsley, mustard, half the oil and pepper to taste. Toss to combine and refrigerate until serving time.
Preheat a nonstick frying pan. Add the remaining oil. Mix the reserved grapefruit juice with the sambal oelek, if using, and brush onto the salmon fillets.
Put the salmon into the frying pan, skin-side-down, and cook over moderately high heat for 2 minutes. Turn the fish over and cook for a further 2 minutes. Serve the salmon with the grapefruit salad alongside.