To make dough, whisk together flour, salt, and sugar. Then add in butter and mix with hands until you have a coarse, crumbly mixture. (Alternatively, you can use a food processor.) Add in water 1 tablespoon at a time until the dough comes together. Be careful not to over mix. Form dough into a disk and wrap in plastic wrap. Refrigerate for at least 1 hour.
Roll out dough on a lightly flowered surface into a 12-inch circle. Place it in a tart pan and press down to fit in the edges. Refrigerate for 30 minutes.
Preheat oven to 375 degrees F. Line dough with parchment paper and fill 2/3 of the way with pie weights or dried beans. Bake until crust is set, about 12-15 minutes.
Carefully remove paper and bake until golden brown, an additional 8 minutes. Remove from oven and let cool on a wire rack.
Meanwhile, make the filling. In a large sauté pan, heat up olive oil over medium heat.
Sauté red onions with a pinch of salt until the start to soften and brown, about 5 minutes.
Then add capers and sauté for another 3 minutes. Set aside to cool.
In a large bowl, whisk together eggs, milk, cream, black pepper and nutmeg. Once crust is cooled, sprinkle half of the onion mixture, goat cheese and smoked salmon on the bottom of the crust. Pour over egg mixture and dollop with remaining ingredients. Bake until quiche is set but still slightly wiggly in the center, about 30 minutes. Let cool for 10 minutes and serve.