To make the crust :
1. Cut the butter into the flour, salt and pepper and rub it through with your hands until flour is grainy.
2. Add ice-cold water, a little at a time, mixing until dough starts holding together.
3. Wrap in plastic wrap and refrigerate for about 30 minutes.
4. Dust a counter surface and rolling pin with flour before rolling out the dough until it’s thin.
5. Drape dough over the baking container, cutting off any excess.
6. Preheat oven to 350°.
To make the onion filling :
7. Saute onion slices in oil and butter until they are caramelized and golden brown.
8. Add the thyme and a pinch of salt.
9. Remove mixture from the plan and allow it to cool.
10. In a bowl, mix the heavy cream, eggs and nutmeg, then add in the onion mixture and season with salt and pepper.
11. Pour mixture into the tart shell and bake 35 to 40 minutes or until the center doesn’t jiggle.