In a big bowl, mix all the ingredients for the crust until well combined. Shape the dough into a ball, cover it with cling film and chill it in the refrigerator while you continue with the recipe.
Add the water to a saucepan and bring it to a boil. Steam your Brussels sprouts for 10-15 minutes or until slightly soft. If you don't have a steam basket you can also quickly parboil them instead.
Meantime, preheat the oven to 425 °F/220 °C. Grease your pie dish with butter, roll out the dough on a lightly floured surface until it's about the hight of a €1 coin. Carefully transfer it to the greased baking pan. Make sure to press the dough into the dish and to pull up the sites by about an inch or a minimum of 2cm.
Place the steamed/cooked Brussels sprouts, pancetta and chestnuts on top of the crust.
In a medium-sized bowl, mix together the remaining ingredients and pour them over the filling.
Bake for about 25 minutes or until lightly browned and set. Cut into 8 slices and serve hot or cold.