Heat a large frying pan over medium-high heat. Spray both sides of the salmon with olive oil spray and season. Sprinkle both sides of salmon with cumin seeds. Cook, skin-side down, for 2 mins or until skin is crisp. Turn and cook for a further 2-3 mins for medium or until cooked to your liking. Transfer to a plate and cover with foil. Set aside for 5 mins to rest.
Meanwhile, combine lime juice, fish sauce, ginger and oil in a small jug.
Combine grapefruit or pomelo, lime segments, cucumber, mint, coriander, red onion and chilli, if using, in a bowl. Drizzle with dressing. Toss to combine. Divide among serving plates. Top with salmon.