Put the shortcrust pastry on a lightly floured surface and roll out until just slightly thinner than the thickness of a £1 coin. Carefully lift the pastry and lightly press it into a 32cm x 21cm tart tin. Place in the fridge to chill for 20 minutes. Preheat the oven to 190ºC, fan 170ºC, gas 5.
Line the chilled tart case with baking parchment, then fill with baking beans (or rice). Bake for 25 minutes, then remove the baking beans and parchment and continue baking for a further 5-10 minutes, until the pastry is crisp and beginning to turn golden.
Meanwhile, lightly whisk together the eggs, soured cream, milk, dill and lemon zest. Season with freshly ground black pepper.
Evenly scatter the salmon trimmings and most of the spring onions into the bottom of the cooked pastry case then pour the cream mixture over the top. Scatter the remaining spring onions and dill sprigs over the top.
Reduce the oven temperature to 180ºC, 160ºC fan, gas 4 and cook the tart for 30-35 minutes, until set in the middle and beginning to turn golden on top. Let it cool completely before transferring to a plate, covering with foil and keeping in the fridge overnight.
Cook's tip: this recipe works just as well with ready-rolled light shortcrust pastry instead.
Save: use basics smoked salmon trimmings.
Cheat: fill a ready-made savoury tart case, or buy a smoked salmon & watercress quiche.
Magic: serve each slice topped with a dollop of half-fat crème fraîche and a few washed and snipped chives.