Preheat oven to 400 . Place prepared pastry into a pie plate and set aside.
Heat oil in a heavy-bottomed, non-stick frying pan over medium heat. Sauté onion until just softened, then add sausage and cook until browned. Add spinach and cook until wilted, then remove from heat and cool.
In a blender, combine eggs and milk and blend for 1 minute. Add cheese, stirring to incorporate.
Pour sausage, spinach and onions into bottom of pie crust. Top with egg and cheese mixture. Bake for 45 minutes to 1 hour, or until eggs are set to desired doneness. Remove the quiche from the oven and rest for 5 minutes before serving.