Roll pastry to the thickness of a coin. Cut six 12-cm (5-in) rounds & fit into six 10-cm (4-in) tartlet tins or Yorkshire pudding tins. Press the dough rounds well into the sides to make sure they come up beyond the tin edges to allow for shrinkage.
Line tart cases w/foil, fill w/baking beans or dried lentils & chill for 20 minutes.
Saute leeks in oil for 5 min till softened & set aside to cool. Preheat oven to Gas Mark 6 (200C or 400F).
Bake tart cases for 10 minutes. Then remove foil & beans (or lentils) & bake for another 5 minutes. Reduce oven temp to Gas Mark 4 (180C or 350F).
Divide leeks + sauerkraut among the tart cases, drop in the crumbled or diced cheese & sprinkle w/the caraway or cumin seeds.
Beat the egg, yolks + milk & pour into the tarts, forking the mixtures to distribute well.
Return to the oven & bake for 20 min till risen & golden brown. Cool slightly b4 removing from the tins & serve warm.