Heat the Oven - Set the oven rack in the middle position. Preheat to 350°F (177°C).
Saute the Vegetables - Heat a large skillet over medium heat. Once hot, add the olive oil. Saute the onions until translucent and lightly browned, about 5 minutes. Add the bell pepper and saute for 1 minute. Add the mushrooms and saute until most of the moisture releases, about 2 to 3 minutes. Add the garlic and saute for 30 seconds. Add the drained, chopped spinach and saute to remove excess moisture, about 1 minute. Season the mixture with ½ teaspoon salt. Add the chopped sundried tomatoes and stir to combine, 30 seconds. Turn off the heat.
Layer the Quiche - Grease a 9-inch pie dish with baking spray or brush with olive oil. Sprinkle half of the mozzarella cheese over the bottom. Transfer the spinach mixture to the pie dish, spreading evenly. Sprinkle the remaining mozzarella cheese and parmesan cheese on top.
Make the Custard - In a medium bowl, whisk together the eggs, heavy cream, whole milk, ½ teaspoon salt, pepper, and nutmeg.
Add the Custard - Slowly pour the custard into the pie dish. Use a fork to move some of the ingredients to help the custard settle. Depending on the depth of the dish, you may not use all of the custard. Sprinkle the surface with black pepper.
Bake - Place the quiche on top of a sheet pan in the middle of the oven until the surface is lightly golden brown, about 30 to 35 minutes. The center should be just set, and the edges slightly puffy. Use a thermometer to measure the temperature. It should be about 190 to 195ºF (88 to 91ºC) in the center and about 195 to 200ºF (91 to 93ºC) on the edges.
To Serve - Cool on a wire rack for 5 to 10 minutes before serving. Slice and serve while still hot.
Notes
Using Fresh Spinach: Roughly chop 6 cups (8 ounces) to replace 10 ounces of frozen. Saute the spinach until wilted. There should be about 1 cup.
Using Pre-Sliced Mushrooms:You'll need 1 ½ cups of sliced mushrooms.
Storing: Store pieces in an airtight container for up to 7 days. To freeze, Individually wrap the slices, then store them in a resealable bag for up to 30 days.
Reheating: Reheat individual slices in the microwave in 15 to 30-second intervals on high power until warmed through.