Heat the oven to 190C (180C fan)/gas 6. Butter the base and sides of a 24cm, wide, 5cm deep tart tin and line the base with baking paper.
Mix the flour, salt and seeds, rub in the butter with your fingertips, or use a freestanding mixer fitted with a paddle, to form a breadcrumb consistency.
Add the egg and milk, and mix to form a dough. Roll out the dough between two pieces of baking paper, roll on to a the rolling pin, unfurl over the tin, press the pastry into the edges of the tin, and rest for 30 minutes in the freezer.
Heat the rapeseed oil in a frying pan. Add the spring onions and ½ tsp salt, cook until soft, then set aside to cool. Mix the milk, creme fraiche, eggs, thyme, 1½ tsp salt and some pepper, then add the cooled spring onions.
Fill the tart case with baking beans or dry rice, and blind bake for 10 minutes, then remove the baking beans, and bake for 10-15 minutes more, until golden brown.
Finally, turn down the oven to 160C (150C fan)/gas 3, brush the case with the egg yolk and bake for two minutes more .Fill the baked case with layers of fish and broccoli, alternating between the two in layers.
Top with the creme fraiche and egg mix, and bake for about an hour, turning the tart halfway through baking, or until just set with a little wobble. Set aside to cool a little, then serve with extra steamed broccoli or lightly dressed watercress.