Heat oven to 220C/200C fan/gas 7. Heat a frying pan with a lid over a medium heat and add the oil. Cook the leeks, with the lid on, stirring occasionally until just tender - about 5 mins. Set aside.
On a lightly floured surface, roll out the pastry and cut out a rough 26cm circle - use a big dinner plate as a template. Cover a baking tray with baking parchment, sprinkle the polenta or semolina over it, and place the pastry circle on top. Score a circle about 2cm from the edge. Spread the leeks out evenly inside the scored circle, then bake for 10 mins until the edges have puffed up. Remove from the oven and turn heat down to 200C/180C fan/gas 6. Bring a medium pan of water to the boil.
Blanch the broccoli in the boiling water for 1 min, then drain and cool under running water. Drain again and arrange on top of the leeks. Mix together the creme fraÃ®che, Parmesan, egg yolks and some seasoning, then pour evenly over the veg. Sprinkle with the flaked almonds. Bake the tart for 15-20 mins until golden. Serve warm or cold.