For the pastry dough
Preheat oven to 180°C. Mix the flour with the butter, until it acquires a crumbly texture. Add the remaining ingredients and mix until a non sticky, soft dough is formed. Roll the dough to be ¼ inch thick and divide into 8 pieces. Place the dough into greased tart molds and prick holes in the dough with a fork, Cut off excess dough from around the edges of the mold with a knife. Cover the dough with foil. Bake in preheated oven for 10 minutes, remove the foil and bake for an additional 10 minutes. Remove from the oven and set aside.
For the onions
Heat the olive oil in a large frying pan, stir in the onions and allow to cook for 10 minutes until soft. Add the salt, pepper, and sugar and continue to cook, stirring until the onions begin to golden. Finally, stir in the vinegar, butter and thyme and let cook for another 10 minutes. Remove the onions from the heat to cool slightly.
Fill each tart with the caramelized onions and sprinkle with grated emmental and parmesan cheese.
Beat the eggs, milk and cream in a bowl. Pour the mixture into each tart mold on top of the caramelized onions. Top with crumble feta cheese and bake for 15-20 minutes at 180°C until cheese melts and turns slightly brown.