Preheat the oven to 350F and grease a 24-count mini muffin pan. See the note below regarding cooking time if using a regular muffin pan.
In a large bowl, whisk the eggs and milk. Add in the chopped spinach and shredded cheese. Mix to combine all ingredients.
Distribute the egg mixture evenly into the muffin pan cups, filling them about ¾ of the way up.
Bake in the preheated oven for 15-18 minutes. Remove the pan from the oven and let them cool in the pan before carefully removing them with a small knife or small spatula.
Once cooled, refrigerate the egg cups in an airtight container or zip bag for up to 3 days.
For lunch:
To enjoy these at room temperature, place 4 to 6 egg cups inside the main compartment of a lunch container.
To eat warm, heat the egg cups in the microwave for 1 minute, stopping the microwave half way through to flip them over. Immediately transfer them inside a thermos container and close the lid.
Finish packing this lunch by adding fruit, veggies, and any other sides inside a bento box. Pack everything inside a lunch bag.
Notes
The cooking time using a mini muffin tray (yields 24) is approximately 16 to 18 minutes. Using a regular-sized muffin tray (yields 12) requires a longer baking time of approximately 26 to 28 minutes.