This recipe combines earthy mushrooms with tender red onion, spinach, Gruyere cheese, and fluffy eggs that are snuggly tucked inside a refrigerator pie crust.
Place pie crust into pie plate and flute the edges. Do not bake. Set aside.
Melt butter in a skillet over medium-high heat. Add garlic and red onion, cooking until tender, about 2-3 minutes.
Add sliced mushrooms, garlic salt, and pepper. Toss to incorporate.
Add spinach, a couple handfuls at a time, stirring occasionally until spinach cooks down and mushrooms are tender. Remove from heat and empty skillet onto a plate. Set aside.
In a large mixing bowl, whisk together the eggs, Dijon mustard, and heavy cream.
Add Gruyere cheese and whisk to combine.
To the pie crust, add half of the spinach/mushroom mixture. Pour egg mixture over the vegetables. Add remaining spinach mushroom mixture over the egg.
Bake for 45-50 minutes. Edges will be browned and center should be set. Remove to wire rack and let cool (and set up) for 7-10 minutes before serving.
Notes
We opted for a refrigerator pie crust to keep things nice and simple. However, you can easily substitute with homemade pie crust if preferred.
It's best to use freshly shredded cheese straight from the block. You can substitute with mozzarella, white cheddar, feta, or Swiss cheese.
Heavy cream is preferable for this recipe as it plays an important role in giving your quiche a creamy, custard-like consistency. You can substitute with half and half in a pinch.
You can use fresh or frozen spinach for this recipe. If you choose to use frozen spinach, be sure to thaw it and pat it dry before beginning. If you add frozen spinach to a quiche or don't pat it dry well enough after thawing, you'll likely end up with a watery quiche.