Steam, microwave or grill the salmon until the flesh flakes easily at the thickest part of the fillet. Cool, then mash with a fork and stir in the smoked salmon, crème fraîche, capers, lemon zest and half of the dill. Season.
Line 6 small ramekins with clingfilm, then spoon in the salmon mix and press it down gently. Cover and chill for at least 1 hour (this can be done a day ahead).
Peel the cucumber, cut in half lengthwise and remove the seeds. Slice thinly, put into a bowl with the vinegar, sugar and red onion, and season. Chill for between 30 minutes and 1 day.
To serve, turn the ramekins onto serving plates and remove the clingfilm. Mix the remaining dill and whole capers into the cucumber relish, then spoon over the top of the rillettes. Serve with lemon wedges and crisp Melba toast (see right, for how to make).
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