Preheat the oven to fan 180C/ conventional 200C/gas 6. Line a 4cm deep, 22cm loose-bottomed tart tin with the pastry, letting the excess hang over the top edge. Prick the base, fill with crumpled foil and bake blind for 15 minutes.
Meanwhile, cook the bacon in a non-stick pan over a fairly high heat for 5 minutes until browned and quite crisp. Remove with a slotted spoon and drain on kitchen paper. Cook the spring onions in the fat over a moderate heat for 1 minute until softened. Remove with slotted spoon and tip over the bacon.
Take the pastry case out of the oven and remove the foil. Reduce the heat to fan 160C/conventional 180C/gas 4. Beat the cream, milk, eggs and egg yolks. Stir in the bacon, spring onions and a little salt, then pour into the case. Scatter with the cheese and black pepper, then bake for 30 minutes until pale golden and just firm to the touch.
Remove from the oven and trim excess pastry with a knife. Leave in the tin to cool a little, then slide on to a wire rack and leave until cold.
For a picnic, make the tart up to 6 hours ahead and return it to the clean tin once it’s cold. Wrap the whole thing in cling film or foil and keep in a cool place (not the fridge or it will get soggy).