Preheat the oven to 350 degrees F. Line a 9-inch pie plate with pie dough and prick with a fork several times. Bake according to instructions, or until lightly golden, about 12-15 minutes. Remove from oven and let cool.
In a medium skillet, heat olive oil over medium heat. Add the onion and asparagus spears and cook until onion is translucent asparagus spears are slightly tender. Stir in garlic and cook until fragrant, about 30 seconds. Remove pan from heat and set aside.
In a large bowl, whisk together eggs, milk, cream, and Dijon mustard until well combined. Season with salt and pepper to taste.
Place the asparagus mixture evenly over the bottom of the cooled pie crust, then sprinkle with feta cheese. Pour the egg mixture over the vegetables and into the crust.
Bake the quiche for 35- 45 minutes or until quiche is set and slightly golden brown. Let quiche stand for 15 minutes before serving.