Pre-heat the oven to 200°C. Roll out the pastry on a lightly floured surface to 1cm thick and cut a round of pastry 3-4 inches larger than an 8 inch straight-sided tart tin. Line the tin with the pastry, pressing in well then prick the base. Trim any overhanging pastry and chill for 10 minutes.
Line the pastry with nonstick baking paper and fill with baking beans. Bake in the oven on a baking sheet for 15 minutes until golden at the edges. Remove the paper and beans at this point, brush the base with 1 beaten egg and return to the oven to allow the base to cook and brown for 5 minutes. Remove from the oven and cool on a wire rack.
Prepare the filling by whisking together the milk and crème fraîche. Whisk in remaining 2 eggs and Gruyère then season well with salt and pepper. Arrange the sliced runner beans in the base of the pastry, then pour the filling on top. Reduce the oven to 170°C and back the tart for 20-25 minutes until the filling is just set. Remove and allow to cool a little before turning out from the tin.
Serve warm or cold.
Freezing and defrosting guidelines
Once the dish has cooled completely, transfer it to an airtight, freezer-safe container, seal and freeze for up to 1-3 months. To serve, defrost thoroughly in the fridge overnight before reheating. Loosely cover with foil and bake until dish is thoroughly heated through. Reheat until piping hot.