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Preheat oven to 350 degrees Fahrenheit.
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Para-bake the pie crust for about 8 minutes. Allow the crust to cool down before adding any other ingredients to it. Put the pie crust on a baking sheet with foil on it. This will save you clean up later.
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In a large pan on medium heat, cook the sausage until it is no longer pink. Add the cooked sausage to the pie crust.
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Lower the heat to a low-medium and add the onions. Cook until the onions soften and begin to caramelize.
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To the same pan, add the butter, the mushrooms, and 1/4 teaspoon of the salt.
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Cook until the mushrooms have reduced in size and have released most of the water. It will take about 8 minutes. We don’t want anything with a high water content in the quiche because it can water it down. We want it creamy, not watery.
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Allow the mushrooms to cool down for a minute. Add the mushrooms to the pie shell.
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Top the mushroom mixture with the cheeses.
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In a bowl, beat the eggs and heavy cream, along with the remaining salt. Pour the egg custard mixture in the pie shell.
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Bake for about 40 to 44 minutes, or until the quiche is set the the top is golden. Enjoy warm or at room temperature.