Preheat the Oven
Turn your oven to 350°F. While it heats up, grease your pie dish with butter. Make sure you coat the bottom and the sides well. This prevents the eggs from sticking to the pan later.
Sauté the Onions
Heat olive oil in a large skillet over medium heat. Add the chopped onions and cook for five minutes. They should be soft and clear. This step adds a sweet base layer of flavor.
Cook the Mushrooms
Add the sliced mushrooms to the skillet with the onions. Cook them for about eight to ten minutes. Wait until the liquid evaporates and the mushrooms turn golden brown. Stir them occasionally for even cooking.
Wilt the Spinach
Add the fresh spinach to the skillet. Stir it constantly until it wilts completely. This should only take two minutes. Remove the pan from the heat. Let the vegetables cool down for a few minutes.
Whisk the Eggs
Crack the eggs into a medium mixing bowl. Add the heavy cream, salt, pepper, and nutmeg. Whisk everything together until the mixture is smooth. Make sure there are no streaks of yellow yolk left.
Combine the Ingredients
Stir the cooked vegetables into the egg mixture. Add the grated Gruyère cheese and the fresh chives. Mix gently with a wooden spoon. Make sure the vegetables and cheese are spread out evenly in the liquid.
Fill the Dish
Pour the quiche mixture into your prepared pie dish. Use a spoon to push the spinach and mushrooms around. You want every slice to have plenty of fillings. Sprinkle a little extra cheese on top.
Bake the Quiche
Place the dish in the center of the oven. Bake for 40 to 45 minutes. The top should be golden and slightly puffed. The center should be set but still have a tiny jiggle.
Cool and Serve
Remove the quiche from the oven. Let it sit on the counter for 10 minutes before cutting. This resting time allows the custard to firm up. Slice it into wedges and serve warm or at room temperature.