To prepare the dough, sift the flour in advance. In a deep bowl, mix it with butter, salt, yolk and ice water. You should get an elastic dough. Wrap it in cling film and put it in the refrigerator overnight.
Place the thinly rolled dough into the pan and trim off any excess edges. Prick the entire surface with a fork and place in the oven under pressure for 20 minutes at 180°C. Cool and bake for another 10 minutes without pressure.
Caramelize the onion in butter until brown. In a separate bowl, mix the fried onion, eggs, and cream cheese. Pour in the cream, add thyme and green onions. Salt and pepper to taste.
Pour the onion mixture into the quiche base, place a sprig of thyme on top and bake at 170°C for 20-25 minutes.