Position rack in centre of oven; preheat to 325°F.
Coat a non-stick small fairy cake tin generously with cooking spray or butter.
Heat a large non-stick skillet over medium-high heat. Add sausage and cook until golden brown, 6 to 8 minutes depending on the number of type of meat used. Transfer to a bowl to cool.
Add oil to the pan. Add mushrooms and cook, stirring often, until golden brown, 5 to 7 minutes.
Transfer mushrooms to the bowl with the sausage. Let cool for 5 minutes.
Stir in scallions/green onions, cheese and pepper.
Whisk eggs, egg whites and milk in a medium bowl. Divide the egg mixture evenly among the prepared cake cups. Place a heap tablespoon of the sausage mixture into each cup. Bake until the tops are just beginning to brown, 25 minutes.
Let cool on a wire rack for 5 minutes. Place a rack on top of the pan, flip it over and turn the quiches out onto the rack. Turn upright and let cool completely.