Combine flour, salt, butter, and lard in a large bowl and rub with fingertips until the mixture resembles coarse breadcrumbs.
Stir in enough cold water to bind the dough together using a knife.
Wrap the dough in cling film and chill in the fridge for 15 to 30 minutes.
Preheat the oven to 190°C.
Roll out the pastry on a floured surface and line a 22cm deep flan tin, allowing edges to overhang.
Chill the pastry case in the fridge for at least 15 minutes.
Line with parchment and baking beans, then bake for 15 to 20 minutes.
Remove beans and parchment, then bake for another 5 minutes before reducing oven to 160°C.
Sauté mushrooms in oil for one minute.
Whisk eggs, milk, and cream in a jug and season with salt and pepper.
Layer cheese and mushrooms in the pastry case, pour over the egg mixture, and sprinkle with thyme.
Bake for 40 to 45 minutes until the filling is set.
Cool the quiche and trim the overhanging pastry edges.
Griddle the spring onions in two tablespoons of olive oil until charred.
Whisk remaining olive oil, mustard, and vinegar to create a dressing.
Toss spring onions, salmon, and watercress with the dressing and serve with quiche slices.