Arrange the oven racks so one is closest to the bottom of the oven. Preheat the oven to 375°F (190°C). Grease a deep, 9-inch/23cm pie dish & line with the pie crust, pressing the crust gently up the side of the pan. Feel free to do a fancy edge. I didn't.
Sprinkle half of the cheese over the base of the pie crust.
Melt 1 tablespoon of butter in a large frying pan over medium heat. Cook, stirring occasionally, for 5 minutes or until onion is softened. Scrape the onion into the pie crust to cover evenly.
Melt the remaining 1 tablespoon of butter in the same pan over medium-high heat with the olive oil. Add the mushrooms, salt & pepper; cook for 2 to 3 minutes each side until browned – don’t over crowd the pan, you might have to cook these in two batches if you don’t have a large frying pan.
Add the garlic & stir to combine. Cook the garlic 1 to 2 minutes until fragrant & softened; remove from the heat. Transfer mushrooms to a large plate lined with paper towel. Or place them in a strainer over a bowl.
Whisk together the eggs, cream, nutmeg, thyme, if you’re using it, and a pinch of salt & pepper in a medium bowl until well combined.
Arrange the mushrooms in the pie crust. Carefully pour in the cream mixture. Sprinkle with remaining cheese. Cook quiche on the bottom rack for 40 minutes or until golden brown & just set. Let cool completely on a wire rack before cutting into wedges to serve. Serve cold or at room temperature with salad greens if desired.