Cook bacon in a skillet over medium heat until the pieces are cooked and crisp, about 6-8 minutes. Remove bacon to a paper towel lined plate. In the same skillet, add the broccoli and onion. Sauté until the vegetables are tender, about 5 minutes. Add in the spinach if using and cook for another minute. Set aside.
Preheat oven to 350°F. Coat a deep 9-inch pie dish with nonstick cooking spray or butter.
In a large mixing bowl, whisk together the eggs, milk, salt, pepper, and ground mustard.
Spoon the bacon and vegetables evenly in the pie dish.
Sprinkle the cheese on top.
Slowly pour the egg mixture into the dish. Sprinkle the green onions on top.
Bake the quiche for about 35 minutes until the center is set. It will look puffed up and golden on the edges. You can check if it is done by using a sharp knife inserted in the center. It should look cooked without any excess liquid.
Remove from oven and let cool for a few minutes before serving.
Notes
You can freeze a unbaked quiche for up to 3 months. Let thaw overnight in the refrigerator before reheating.This can be made ahead of time. You can fully assemble the unbaked quiche and keep it covered in the refrigerator until you are ready to bake it. Let it sit at room temperature for about 30 minutes and then bake as directed.