Start by preheating your oven to 350°F (175°C). Use a high-quality non-stick cooking spray or a small amount of butter to grease a standard 12-cup muffin tin. Ensuring the tin is well-greased is vital for making sure the egg cups pop out easily after baking.
In a medium-sized mixing bowl, crack the six eggs. Add the milk, salt, and black pepper. Whisk the mixture vigorously until the yolks and whites are fully combined and the mixture looks slightly frothy. This airiness helps create a lighter texture in the finished egg cups.
If you are using spinach and bacon bits, fold them into the egg mixture now. Stir gently to distribute the ingredients evenly so that every egg cup gets a bit of everything. You can also add other finely diced vegetables at this stage if you prefer.
Carefully pour the egg mixture into the prepared muffin tin cups. Aim to fill each cup about two-thirds to three-quarters of the way full. Do not fill them to the very top, as the eggs will puff up while they cook in the oven.
Sprinkle a generous amount of shredded cheddar cheese over each cup. Follow this with a hearty dusting of the Everything Bagel Seasoning. Place the tin in the center rack of the oven and bake for 20 to 25 minutes, or until the centers are set and the tops are slightly golden. Let them cool for a few minutes before removing them from the pan.