1. First prepare the gravlax. Mix together sugar, salt, fennel seeds, chopped dill and mustard.
2. Spread the mustard mixture evenly over both sides of the salmon fillet, place it in a large dish, cover with plastic wrap and leave to marinate in the refrigerator overnight.
3. Preheat the oven to 350°F
4. Roll out the pastry dough to 1/4in thickness and cut discs, using a 4 3/4in circular cutter.
5. Place the pastry discs on a greased baking sheet and bake for 10-15 minutes until crisp and golden. Remove from the oven and allow to cool.
6. To make the dressing, mix together the grain mustard, vinegar, sugar, olive oil and dill. Add a little water to thin the dressing, if necessary.
7. Using a sharp knife, slice the marinated gravlax very finely.
8. Place the gravlax slices neatly on the pastry discs. Drizzle over the dressing, garnish with fresh dill and serve at once.