Heat oven to 200C/180C fan/gas 6. Roll out the pastry and use to line a deep 20 x 30cm loose-bottomed tart tin or 23cm round tin. Prick the base and line with greaseproof paper and baking beans. Sit the tin on a baking sheet and bake for 15 mins, then remove beans and parchment and return to the oven for a further 5-10 mins until pale golden. Lower the oven to 180C/160C fan/gas 4.
Meanwhile, trim the cabbage and separate the leaves. Cook in a pan of lightly salted boiling water for 4-5 mins, until tender. Cool and drain well then squeeze out any excess water. Pat dry with kitchen paper.
Arrange the cabbage leaves and pancetta slices in the tart case. Beat together the milk, cream and eggs until well blended. Stir in the Parmesan and some freshly ground black pepper then pour into the tart case. Bake for 25 mins until just set.
Leave to sit in the tin for 10 mins then carefully lift out, cut into slices and serve while still warm.