Heat the butter in a large frying pan and gently sauté the onions over medium heat. Cover them for the first 15 minutes to get them steaming and releasing their moisture, then uncover, reduce the heat to low, stir in the shredded leeks and continue cooking until they are cooked down but not overly brown, 45 minutes to 1 hour. Set aside and let cool when they’re finished.
Preheat your oven to 180°C/160C. Roll out the dough to a thickness of about ¼ inch. Butter and lightly flour a 9” lose bottom tart tin. Lay the dough into the base and fit it snugly, there should be plenty of dough overhanging the edges to help it maintain its shape. Reserve a small piece of dough to fill any cracks that might open in the dough as it bakes.
Blind bake the dough until the crust is golden brown: line the dough with parchment and fill it with dried beans. After about 25 minutes, remove the beans and parchment, gently patch with the reserved dough any cracks that may have formed, and continue baking until the bottom of the crust is golden and cooked, about 10 minutes more. Remove the crust from the oven and patch any cracks that may have opened. Reduce the oven temperature to 165°C/145 fan.
In a large bowl whisk together the milk, cream, eggs, salt, pepper and nutmeg, whisk vigorously until frothy. The idea will be to add the ingredients in two layers, using the froth to help keep the ingredients suspended.
Layer half of the onion and leek mixture into the shell. Pour half the frothy custard over the mixture. Sprinkle with half the cheese. Layer with the remaining onion-leek mixture. Re-froth the batter and pour the rest into the shell. Sprinkle the remaining cheese over the top.
Bake in the 165°C/140C fan oven for about 1 hour, or until the centre is just set. It may take as long as 1½ hours, but don’t overcook it – there should still be a slight wobble in the centre.
Cool slightly before slicing and serving- delish!
