Preheat your oven to 375°F (190°C). This ensures your quiche cooks evenly.
In a large skillet over medium heat, add the ground beef. Cook until browned, breaking it apart with a spatula. This should take about 5-7 minutes. Once cooked, drain any excess fat and set aside.
In the same skillet, add the sliced mushrooms. Sauté for about 5 minutes until they are tender and have released their moisture. Season with a pinch of salt and pepper.
In a large mixing bowl, whisk together the eggs, heavy cream, whole milk, sea salt, and cracked pepper until well combined.
Add the cooked ground beef, sautéed mushrooms, chopped spinach, green onion greens, and shredded Swiss cheese to the egg mixture. Stir gently to combine all ingredients.
Grease your baking dish lightly with oil or cooking spray. Pour the mixture into the dish, spreading it evenly.
Place the baking dish in the preheated oven and bake for 35-40 minutes, or until the quiche is set in the center and lightly golden on top.
Once done, remove from the oven and let it cool for about 10 minutes. This will help the quiche firm up for easier slicing. Serve warm or at room temperature.
