To make the crust, combine the flour, sugar, and salt in a bowl. Add the butter and blend into the flour mixture with a pastry cutter or your fingers until the butter is the size of small peas. Add the buttermilk and mix with your hands or a spoon until it comes together into a ball. Add more buttermilk a tablespoon at a time if necessary, but it's okay if it's a little on the dry side, chilling will bring it all together.
Divide the dough in half and wrap in plastic wrap. You'll need one ball of dough for this quiche, save the rest for your next pie adventure! Refrigerate the other half for up to one week or freeze for up to several months. Chill the dough you will be using for at least 2 hours or overnight.
On a lightly floured surface, roll the pie crust into a 11 inch circle and transfer to a greased 9-inch tart pan with a removable bottom. Prick the bottom of the crust with a fork and line with a greased sheet of aluminum foil. Fill the pie crust with baking beans or pie weights. Chill in the freezer while you preheat the oven to 400°F. Once the oven is ready, bake the pie crust for 10 minutes, then remove the foil and bake for an additional 5 minutes until it just begins to brown.
Reduce the heat of your oven to 375°F.
To make the filling, wash and dry the spinach. Roast the peppers. You can do this over a gas stove or in a 400°F oven on a foil lined sheet pan for 45 minutes until charred. Place the roasted peppers in a covered container for 15 minutes, then peel the charred skins off with your hands. Do not rinse the peppers as that will wash all the flavor away. It's okay if a few black bits remain. Cut the peeled peppers in half, remove the juices, seeds, and stem, and thinly slice.
In a medium saute pan, heat the olive oil over medium heat. Add the spinach and toss until it just begins to wilt. Spread the spinach, sliced peppers, and crumbled feta cheese into the prepared pie crust.
In a bowl, whisk together the whipping cream, eggs, salt, and pepper. Pour into the filled crust. Bake for 30 to 35 minutes until the quiche is set and the top is a light golden brown. Let sit for a few minutes before removing from the tart pan. Serve warm or at room temperature.
Store in the refrigerator.