Preheat the oven to 180°C (160°C fan oven) Gas Mark 4. Roll out the pastry on a lightly floured surface until big enough to line a shallow, 20cm loose-bottomed tart tin. Line the pastry with foil or greaseproof paper, fill with baking beans and bake for 25 minutes. Remove the paper and beans and cook for 10 minutes more until golden.
Scatter the crab meat, spring onions and dill evenly into the pastry case. Whisk together the whole eggs, yolks, sour cream and seasoning and pour into the tart case. Arrange the whole asparagus spears on top, then bake for 35-40 minutes until just set. Serve the tart warm or cold, scattered with a little extra dill.