Preheat oven to 375 degrees Fahrenheit, and place your baking rack on the bottom slot of your oven.

Unroll your pie crust and press it gently into a 9-inch pie plate or tart pan. Either tuck in any excess crust and crimp with a fork, or use your fingers to make a decorative border by pinching the dough between the pointer finger of one hand and the thumb and pointer of the other hand as you move around the circle.

Use a fork to prick the crust all over the bottom—this will make sure the bottom doesn’t puff up as it bakes.

Line the crust with parchment paper or aluminum foil and cover with pie weights, uncooked rice, or dried beans. Bake on the bottom oven rack for 20 minutes, then remove the parchment and weights. The edges of the crust should be golden, but the bottom will still look a little raw.

Bake for an additional 5 minutes, until the bottom dough no longer looks raw.

Meanwhile, while the crust is baking, clean the mushrooms. Slice mushrooms (cut in half as well if they are large), and mince garlic clove. Peel and slice onion.

Heat 1 tablespoon of olive oil in a medium skillet over medium-high heat. Add sliced mushrooms and sauté until they lose their moisture and start to brown, stirring frequently, about 11-14 min. Season lightly with salt and pepper while cooking.

Add garlic clove and sauté for 1 minute. Scrape mushrooms and garlic into a bowl and set aside.

Add ½ tablespoon olive oil, then add the onion slices. Sauté onion for 5-7 minutes, until softened and lightly browned. Add to bowl with mushrooms and garlic, and stir.

In a different medium bowl, whisk together eggs, cream, mustard, salt, and pepper.

When crust has been removed from oven, spread the mushroom/onion filling over the crust, then sprinkle the cheese over it. Pour in the egg mixture.

Bake for 25 minutes on the bottom oven rack, or until the egg custard is cooked through and golden brown on top. The filling should be completely set—if it jiggles when you touch the pan, it’s not done. Let cool, then serve either warm or at room temperature.