Start by preheating your oven to 350°F (175°C). This will ensure that your quiche cooks evenly.
In a large skillet over medium heat, add the ground beef. Cook it until browned and fully cooked through, breaking it up with a spatula as it cooks. Drain any excess fat and set the beef aside.
In the same skillet, add the sliced mushrooms. Sauté them until they are tender and have released their moisture, about 4-5 minutes. Add the chopped spinach and cook for an additional 1-2 minutes until wilted. Remove the skillet from heat.
In a large mixing bowl, whisk together the eggs, heavy cream, whole milk, sea salt, cracked pepper, and crushed red pepper (if using). Mix until smooth.
Add the cooked ground beef, sautéed mushrooms, and spinach to the egg mixture. Stir in the shredded Swiss cheese and green onion greens, ensuring everything is evenly distributed.
Pour the mixture into a greased 9-inch pie dish or baking dish. Bake in the preheated oven for 35-40 minutes, or until the quiche is set and lightly golden on top. A knife inserted in the center should come out clean.
Once baked, remove the quiche from the oven and allow it to cool for about 10 minutes. This will make slicing easier. Serve warm or at room temperature, and enjoy every savory bite!