Step 1: Preheat your oven to 425°F (220°C).
Step 2: In a large mixing bowl, combine the thawed hash browns, melted butter, 1/2 teaspoon salt, 1/4 teaspoon black pepper, and 1/2 cup shredded cheddar cheese. Mix everything thoroughly.
Step 3: Firmly press the seasoned hash brown mixture into the bottom and up the sides of a greased 9-inch pie dish or quiche pan, creating an even, compact layer.
Step 4: Place the hash brown crust in the preheated oven and bake for 20-25 minutes, or until the edges are golden brown and crispy. Remove from oven and cool slightly.
Step 5: In a separate mixing bowl, whisk together the 6 large eggs and 1 cup milk until well combined and smooth. Season generously with salt and pepper to taste.
Step 6: Stir the chopped fresh spinach, diced bell peppers, sliced green onions, and 1 cup shredded Swiss or mozzarella cheese into the egg mixture. If using, add the optional cooked bacon or ham.
Step 7: Carefully pour the vibrant egg and vegetable mixture into the pre-baked hash brown crust, spreading the filling evenly.
Step 8: Return the assembled quiche to the oven and bake for an additional 30-35 minutes. The quiche is ready when the egg filling is set and the top is lightly golden.
Step 9: Allow the quiche to cool for about 10 minutes before slicing and serving. Garnish with extra green onions if desired, and enjoy warm or at room temperature.