Add the mushrooms to the cast iron skillet and cook about 5 minutes, stirring occasionally, until browned and tender. Next, add the onion and garlic; cook another 5 minutes, stirring often, until softened and tender. Lastly, add the spinach; cook for about 1 to 2 minutes, tossing constantly, until wilted. Remove the cast iron skillet from the EGG.
Set the EGG for indirect cooking with the convEGGtor at 375ºF/191ºC.
Prepare the pie crust to a cool cast iron skillet. Whisk together eggs, milk, half-and-half, Dijon mustard, thyme, salt and pepper in a bowl. Stir in half of the mushroom mixture and 1 1/2 cups cheese. Spoon the mixture into pie crust. Add the reserved half of the mushroom mixture to the top of the egg mixture.
Bake for about 30 minutes or until set and golden brown. Let cool for 10 minutes before slicing. Garnish with thyme and ½ cup of cheese and serve. Enjoy!