Roll out the pastry on a lightly floured surface to a circle roughly 35cm in diameter. Use the rolling pin to pick up the pastry, then drape it over a 27cm deep, loose-bottomed tart tin. Press the pastry into the sides, then prick the base with a fork. Cover with cling film, then chill in the fridge for at least 30 mins.
Heat oven to 200C/fan 180C/gas 6. Unwrap the tart and line with baking parchment and baking beans. Cook on a baking sheet for 10-15 mins, until just starting to firm up. Beat 1 of the eggs. Remove the beans and baking parchment, then lightly brush the tart all over with the beaten egg. Cook for another 5-10 mins until the pastry is golden and crisp all over, then leave to cool.
Reduce the oven to 180C/fan 160C/ gas 4. Whisk together the remaining eggs, then pour in the cream and lemon zest and whisk until well combined. Season with salt and pour into a large jug. Pour the cream mixture into the tart case, then arrange the asparagus spears on top. Dollop over teaspoonfuls of ricotta and sprinkle with the mint, then carefully slide into the oven. Cook for 25-30 mins until the quiche is set with just the faintest wobble in the middle. Leave to cool at least 10 mins before serving either warm or at room temperature, sprinkled with salt flakes.