Prepare the pie crust - Preheat your oven to 350°F (175°C). Roll out the pie crust and transfer it to a 9-inch pie dish. Gently press the crust into the dish, ensuring it covers the bottom and sides evenly. Trim any excess crust hanging over the edges. - If you prefer, you can use a store-bought pie crust for convenience. Make sure to follow the package instructions for baking.

Saute the mushrooms and onions - Heat a tablespoon of oil in a frying pan over medium heat. Add the sliced mushrooms and diced red onion, season with salt and pepper, and saute until the mushrooms are golden brown and the onions are tender, about 5-7 minutes. Remove from heat and set aside. - If you prefer a milder onion flavor, you can also use raw red onion in the quiche instead of sauteing it. Simply add the raw onion to the quiche filling.

Prepare the quiche filling - In a mixing bowl, whisk together the eggs, heavy cream, salt, and pepper until well combined. Stir in the sauteed mushrooms, onions, and shredded cheese. - For additional flavor, you can add some fresh herbs like parsley or thyme to the quiche filling.

Bake the quiche - Pour the quiche filling into the prepared pie crust. Place the quiche on a baking sheet and transfer to the preheated oven. Bake for 35-40 minutes or until the quiche is set and golden brown on top. Remove from the oven and let it cool slightly before serving. - To check if the quiche is fully set, gently shake the baking dish. If the center jiggles slightly, it needs more baking time.

Serve and enjoy - Slice the Mushroom, Red Onion, and Cheese Quiche into wedges and serve warm. It can be enjoyed as a delightful breakfast, brunch, or even a light dinner. Pair it with a fresh salad for a complete meal. -