In a large bowl, whisk together the flour, salt and chives. Add the cold diced butter and rub with your fingers (or a fork or pastry cutter) until it resembles coarse crumbs the size of peas. In a small bowl, lightly whisk the egg, egg yolk and buttermilk together. Pour into the center of the dry ingredients and mix with a spatula until the dough comes together. Use your hands to form into a disk. Cover tightly with plastic wrap and chill in the fridge for about 1 hour.

Preheat oven to 350°F.

On a lightly floured work surface roll out the crust dough to a large circle (enough to cover and fit into a 9-inch tart pan). Place into a removable bottomed tart pan or a deep dish pie plate. Trim the excess hanging dough and even out the top with the edge of the pan or crimp the dough, if using a pie dish. Poke holes throughout the top with a fork. Cover with a piece of parchment paper and fill with dry beans or pie weights. Bake for about 20 minutes covered. Carefully remove the beans or weights and parchment paper. Bake for another 10 minutes or until lightly golden brown. Remove from oven  and allow to cool.

Bring a large pot of water to a boil. Lightly season with salt and throw in the cleaned and trimmed Brussels sprouts. Boil for about 5 minutes or until bright green and tender. Using a slotted spoon transfer the Brussels sprouts to a bowl of ice water. Leave there to cool completely. Then drain into a colander and allow to air dry.

Heat a large skillet over medium-high heat. Melt the butter and add the sliced shallots and minced garlic. Cook for about 5 minutes, stirring constantly, until tender and just beginning to brown. Add the drained Brussels sprouts, stir and cook for about 5 to 8 minutes or until they brown. Season with salt, black pepper, crushed red pepper flakes and nutmeg. Stir every once in a while, keeping an eye on them so that they don't burn. You might have to lower the heat. Add the diced apples and cook for another 2 to 3 minutes. Remove from flame and allow to cool.

In a large bowl whisk together the eggs and half and half. Season with salt and pepper and whisk once more until completely combined. Arrange the sautéed veggies on the baked and cooled crust on a single layer. Pour the milk and egg mixture over the filling. Sprinkle with shredded gruyere cheese. Place the quiche on a baking sheet and bake for about 30 to 35 minutes or until puffed and golden brown. Remove from the oven and allow to cool for about 30 minutes to set. Cut and enjoy!