Stone and peel the avocados, cut into chunks and toss in half the lemon juice. Twist and fold the smoked salmon pieces onto serving plates, then scatter with the avocado. Cover and chill for up to 1 hr until ready to serve.
Mix together the tarragon, crème fraîche and remaining lemon juice. Drizzle over the salmon, scatter with the capers and serve straight away with wholemeal or soda bread.