Preheat oven to 190°C (170°C fan).

Fit the pastry in a lightly greased 20cm tart tin with removable bottom. Prick base all over with fork. Line with foil and refrigerate for 30 minutes. Fill the foiled-lined pastry case with baking weights or dried beans and bake for 10 minutes. Remove the foil and bake a further 10 minutes, or until the pastry is golden and set.

Turn oven down to 180°C (160°C fan). Place the cornflour in a small bowl and mix well with 2 tablespoons of water. Set aside.

Melt the butter in a frying pan, add the fennel and cook over a moderately low heat for about 10 minutes or until softened. Add wine and sour cream and season with salt and pepper. Cook for a further 10 minutes, or until the sauce thickens slightly. Add the cornflour mixture and cook for a further 5 minutes, or until the fennel is coated in a rich sauce.

Spoon into the prepared tart case, cover with gruyère and bake for 30 minutes, or until tart is puffed and golden.

Serve warm or at room temperature.