1. Preheat the oven to 200°C/400°F. Roll out the puff pastry on a lightly floured surface to a thickness of about 3mm/1/8 inch. Transfer the pastry to a 23cm/9 inch tart pan with a removable bottom. Trim the edges and press the pastry into the corners of the pan.

2. In a large bowl, combine the smoked salmon, aged cheddar cheese, chopped onion, and minced garlic. Mix well to combine.

3. In a small bowl, beat the egg and mix with a fork. Brush the egg mixture around the edges of the pastry.

4. Spoon the salmon and cheese mixture into the center of the pastry, leaving a 2.5cm/1 inch border around the edges.

5. Fold the edges of the pastry up over the filling, pressing gently to seal. Brush the pastry with the beaten egg and sprinkle with chopped fresh dill.

6. Drizzle the olive oil over the filling and season with salt and pepper to taste.

7. Bake the tart for 25-30 minutes, or until the pastry is golden brown and the filling is heated through.

8. Remove the tart from the oven and let cool for a few minutes before serving.