Preheat the oven to 180°C, fan 160°C, gas 4. Cut the bread into 6 thin slices, then cut each slice into 4 small triangles. Spread the triangles out on a large baking sheet, drizzle over the olive oil and season with freshly ground black pepper.
Bake for 5 minutes, then turn the triangles over and bake for a further 5 minutes. Remove from the oven and set aside.
Put three quarters of the smoked salmon in a food processor with the soft cheese, horseradish and lemon juice. Whizz until you have a smooth pâté, then transfer to a bowl. Stir in the rest of the smoked salmon and the finely snipped chives. Season with freshly ground black pepper.
Top each crouton with a small dollop of the pâté. Use a vegetable peeler to cut the cucumber into thin ribbons and arrange them on top of the pâté. Garnish with the extra chives, season with freshly ground black pepper and serve on a platter with the lemon wedges to squeeze over.