Preheat oven to 350°F (180°C).
To make pastry: In a large bowl, combine flour and salt. Using two knives, cut butter into the flour and mix until ingredients resemble coarse meal. Add the cold water a little at time, gently stirring until incorporated. Knead dough briefly to form a smooth ball. Cover with plastic wrap and refrigerate for 1 hour.
Remove dough from refrigerator and place on a lightly floured surface. Using a rolling pin, roll out the dough until ¼-inch thick. Press into a pie plate, allowing the dough to hang over the edge. Trim dough evenly so its edges are just slightly over the edge of the pan.
Poke the bottom of the dough with a fork. Place into the oven and bake for 15 minutes.
Remove the partially cooked pastry from the oven and set aside.
To make filling: Chop potato into small, bite-size pieces. Transfer to a bowl and mix with chopped onions. Scatter the potatoes and onions evenly onto the pastry.
In a medium bowl, whisk together the eggs, milk, thyme and goat cheese. Slowly pour the mixture over the potatoes and onions. Place into the oven and bake for 40 minutes, until custard has hardened.
Remove quiche from the oven and allow to cool. Slice into 8 pieces. Garnish with arugula and prosciutto. Serve and enjoy!
