Cooking fish correctly is simple, as long as you remember that it will continue to cook once it's removed from the pan. Salmon is best if it is slightly undercooked, as this gives the best texture and taste.
1.Cut salmon crossways into 8 pieces; place in a large bowl with oil and thyme, season. Turn salmon to coat in mixture; Refrigerate 1 hour.
2.Combine crème fraîche, rind and 1½ tablespoons of the juice in a small bowl.
3.Heat a lightly oiled large frying pan over high heat. Add remaining juice to salmon; toss to combine. Drain salmon from marinade. Cook salmon, in batches, for 2 minutes each side (medium rare) or until cooked as desired.
4.Spread crème fraîche mixture on plates. Top with salmon, watercress and salmon roe.
Salmon can be refrigerated for up to 6 hours to develop the flavour. Ask the fishmonger to remove the pin bones from the salmon for you. Salmon roe is available from fishmongers and large supermarkets (substitute finely chopped chives or dill for the roe, if you prefer). Watercress can be trimmed the day before; soak it in cold water for a few minutes, then drain, and place it in a plastic container lined with a few sheets of paper towel. Refrigerate until ready to serve. The watercress will remain fresh for several days if stored like this.