1. Preheat the oven to 180°C/160°C fan/Gas 4. Remove the skin from the mackerel fillets, cut them in half lengthways and flake the flesh, removing any bones. Scatter the mackerel and watercress over the base of the ready-made tart shell.
2. Beat together the eggs, milk, cream, mustard and some salt and pepper.
3. Carefully pour the egg mixture over the filling, stopping when the liquid reaches the
rim of the case, be careful not to let it overflow. Top with grated cheese then bake for 30-40 minutes, or until the filling has set.
4. Remove the tart from the oven. Trim off any excess pastry around the rim, allow to cool.
5. Cut into wedges and serve.