Slice the onions into small, thin,
wedge-shaped pieces. Heat the
rapeseed oil in a large, preferably
non-stick, sauté or deep frying pan. Stir
in the onions and fry over a medium heat
for about 20 mins. Stir only occasionally
so they can cook fairly undisturbed and
caramelise underneath. When done, they
should be well caramelised and sticky.
Meanwhile, prepare the base. Heat
oven to 190C/170C fan/gas 5 and put in
a baking sheet. Put the flour and butter
in a bowl with a pinch of salt and rub
with your fingers until it resembles rough
crumbs. Mix the yogurt and milk, pour it
into the flour mixture and work together
briefly with a knife until the dough just
comes together. Remove from the bowl
and gently press together to form a ball.
Do not overwork the mixture.
Roll dough out thinly on a lightly floured
surface and use to line a 23cm round,
fluted, loose-based flan tin, about 2.5cm
depth, pressing it with your fingers into
the flutes. Sit the tin on. If the dough is a little sticky,
dust your fingers in some flour.
Beat the eggs in a medium bowl, then
stir in the crème fraîche, thyme, mustard
and seasoning. Spoon and spread half
of the onions into the lined base, season
with pepper then scatter over half the
cheese. Pour the egg mixture over the
top, then scatter over the rest of the onions and cheese. Sit the tin on
the hot baking sheet and bake for 25-30
mins until the base and filling are cooked.
Remove and cool for about 10 mins,
then remove from the tin and serve
while fresh and warm.