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Heat oven to 200C/180C fan/gas 6. Put a baking sheet in the oven to heat up. Dust a clean work surface with flour, then roll out 600g of the pastry to line a 20 x 30cm tin. Trim the edges with scissors so that the pastry sits 2-3mm higher than the sides of the tin. Prick the surface all over with a fork, then line with baking parchment and add an even layer of baking beans. Place on the hot baking sheet and blind-bake for 15 mins.
Meanwhile, roll out the remaining pastry and any off-cuts, cut into strips and plait together. Take the tin out of the oven, remove the baking beans and brush all over with a thin layer of beaten egg. Stick the pastry plaits onto the edges, brush with beaten egg and return to the oven for another 15 mins until cooked through but not brown.
Meanwhile, heat the butter in a frying pan over a low heat and add the onion. Cook gently for 15-20 mins or until soft, then stir in the fennel seeds and take off the heat. Leave to cool in the pan.
Remove the tart case from the oven and set aside while you cook the rest of the filling. Add the asparagus to a large sauté pan, cover with 2-3cm water, bring to a simmer and cook for 5 mins. Drain, then rinse in cold water to cool down quickly. Leave to dry on kitchen paper. Put the empty pan back on the hob, add the milk and salmon, and bring to a gentle simmer. Cook for 4 mins, turning once.
Carefully take the salmon out of the milk and lay on a plate. Pour the milk into a large heatproof jug and set aside. Spoon the onion and fennel seeds in an even layer across the base of the pastry case. Add the asparagus and, when the salmon is cool enough to handle, break chunks of it over the asparagus. Add the cream to the milk along with the rest of the eggs, the dill and lemon zest, then season. Pour the mixture into the pastry case, top with the cheese, then carefully put back in the oven for 45 mins-1 hr or until the filling is set and the cheese on top is turning golden. Serve warm or cold with a fennel and rocket salad.