Warm a frying pan over a medium heat. When hot, tip in 1 tsp coriander seeds and dry fry for 2-3 mins till popping and fragrant smelling. Transfer to a pestle and mortar and lightly crush. No pestle and mortar? Tip the seeds into a bowl and crush with the end of a rolling pin.
Cut the ends from the grapefruit so it stands upright. Carve the skin away, removing as much pith as possible. Cut the grapefruit into segments over a bowl to catch any juice. Put the segments in a separate large bowl.
Pour 1 tbsp each of honey and mustard into the grapefruit juice bowl. Whisk together with some seasoning and the toasted coriander seeds.
Peel and finely chop the onion. Heat the frying pan you used for toasting the coriander seeds. When warm, add 2 tsp oil. Add the onion. Season and cook for 3-4 mins till just soft.
Drain and rinse the lentils. Add them to the onion pan. Stir for 2 mins till warmed through. Transfer to the bowl with the grapefruit segments and cover.
Wipe the pan clean if you need to. Rub 1 tsp oil onto each trout fillet. Season. Heat the pan again and place the trout fillets in, skin-side down. Cook for 3 mins till the skin is crispy and the fish nearly cooked through. Cover and cook for a further 1 min to finish cooking the fish. It should flake easily when pierced with a fork.
Pour the honey and mustard dressing over the lentils. Toss together with a handful of the watercress.
Pile the lentil and grapefruit salad onto 2 plates. Top with the pan fried trout and scatter over the remaining watercress.