If making your own pastry, blitz the flour and a good pinch of salt together in a food processor. Add the butter and blitz until the mixture resembles breadcrumbs. Pour in the milk and blitz until just coming together, then tip onto the worktop, bring together with your hands, and form into a disc. Wrap in clingfilm and pop in the fridge for 30 mins.
Roll out the pastry on a floured surface and use to line a 23cm x 3cm loose-bottomed sandwich tin. Leave the excess pastry hanging over the sides and prick the base with a fork. Pop on a tray and transfer to the freezer for 30 mins.
Heat oven to 200°C/180°C fan/gas 6. Line the pastry case with baking parchment, fill with baking beans and bake for 15 mins.
Meanwhile, to make the filling, heat the oil in a frying pan over a medium heat and soften the spring onions for 4 mins. Add the peas and cook for a further 2 mins, then remove from the heat. Whisk together the eggs, cream, milk, and some seasoning in a large jug. Reserve the tips of the mint leaves for garnishing and finely chop the rest, then stir into the cream mixture.
Remove the beans and parchment paper from the pastry case and return to the oven for 10-15 mins until the pastry is golden. Using a sharp knife, trim the edge. Reduce the oven temperature to 180°C/160°C fan/gas 4.
Arrange half the pea mix and half the Prosciutto di Parma over the base of the pastry case, then top with half the feta. Pour over the cream mix, then scatter over the remaining peas, Prosciutto di Parma and cheese. It will be very full.
Bake for about 35-45 mins, or until golden and with a little wobble in the centre, which should not feel too firm. Leave to cool for 15 mins, then remove from the tin.
Serve warm or at room temperature, topped with the pea shoots and reserved mint leaves.