Make the shortcrust pastry (see recipe below).
You can also watch how to make the pastry in our Cookery School Video on the right. Begin by peeling and slicing the red and white onions in half, then slice thinly so that you get half moons.
Now chop the sage leaves finely, reserving about 8 whole leaves for later. Next in a large, thick-based saucepan melt the butter then add the sliced onions and, keeping the heat highish, toss them around until they are lightly tinged brown at the edges – which will take about 5 minutes.
Now turn the heat down to low and leave them to cook slowly, stirring from time to time, for about half an hour to 40 minutes, by which time they should be meltingly brown. Then stir in the chopped sage and some salt and freshly-milled pepper. While that’s happening roll out the pastry and line the flan tin with it, making sure you leave about ¼ inch (5 mm) sticking up above the rim (to allow for shrinkage). Prick the base all over with a fork and pre-bake it for 10 minutes.
Now beat up the eggs, and brush the pastry base with a little of the beaten egg, then let it cook for a further five minutes. Meanwhile chop the spring onions (including the green parts) quite small. Remove the pastry from the oven and sprinkle in the spring onions, followed by the cooked onions. Next beat the eggs into the crème fraiche with some seasoning, and pour all over the onions – you will need to spread it out evenly and don’t worry if it looks a bit thick, it will melt and surround the onions during the cooking.
Finally sprinkle the Parmesan all over and cook it for 30 minutes on the centre shelf of the oven.
Then brush each reserved sage leaf with a little oil and arrange them on top of the tart, and return it to the oven for another five minutes or until the centre feels springy.
Leave it to rest for 10 minutes before serving.
Pre-heat the oven to 180C, gas mark 4
Using a fan-assisted oven? Click here