Preheat the oven to 160°C (gas 3). Thoroughly grease a 750ml loaf tin with the melted butter.
Put the salmon in a blender or food processor with the eggs, milk, mixed spice, brandy, Tabasco and chopped tarragon. Blend together, then season with salt and pepper.
Pour the mixture into the loaf tin, cover with buttered foil and place in a roasting pan. Pour sufficient boiling water into the pan to come about half way up the sides of the loaf tin. Bake for 40–50 minutes until set – insert a knife point and it should come out clean. Leave to cool in the tin, then place in the fridge and chill overnight.
Remove the terrine from the fridge 30 minutes before serving so that it isn't too cold. Just before serving, prepare the sauce. Add a pinch of salt to the cream and beat until thick. Blend in the lemon juice and tarragon.
Carefully run the blade of a knife all around the inside of the tin, then turn the terrine out onto a long platter, so that it comes out cleanly. Slice and arrange on individual plates. Place a small spoonful of dressing on each serving, then offer the remaining sauce in a separate dish. Serve with salad leaves and garnish with tarragon sprigs.
You can buy fresh tarragon during the summer months. Even better, grow some yourself and freeze for year-round use.