1. Preheat oven to 350°F.
2. Heat up olive oil in a large frying pan over medium heat.
3. Add chopped onions, mushrooms, salt and pepper. Stir occasionally until they start browning (watch closely so they don’t burn). This should take about 15 minutes total.
4. In a large bowl, add the eggs, yolks, and milk. Whisk until yolks are broken and the mixture is pale yellow.
5. Add half the shredded cheese into the bottom of the pie crust followed by the caramelized vegetables. Pour custard mixture carefully over the top of the filling. Top with remaining cheese.
6. Bake on a baking tray (for stability when using frozen pie shells) for 35-40 minutes. A knife inserted into the center of the quiche will come out clean.
7. Allow quiche to set for one hour before slicing.

Recipe Notes: Store leftovers in an airtight container in the fridge for up to 3 days.