Preheat cooking pan over medium heat and add oil. Saute diced onion until transparent and add sausage. Break apart sausage as much as possible. Cook, stirring often until sausage is completely done. Take off heat.
Preheat oven to 350°. Grease deep dish pie pan and gently fit pie crust it the pan.
Whisk eggs, sour cream, heavy whipping cream, salt, and pepper until smooth. Stir in 2/3 of the cooked sausage mixture.
Pour in the eggs and sausage mixture into the pie pan, make sure it's spread evenly. Place on a baking sheet and bake for 40-45 minutes.
Heat up remaining sausage over medium heat. Sprinkle flour over it. Stir well until all sausage is coated in flour.
Start pouring in milk while stirring constantly but slowly.
Keep cooking and slowly stirring until thickened. Take off heat.
Top off each slice of quiche with some gravy when servings.