Preheat your oven to 375 F.
Prepare 6 wells of a muffin tin with A LOT of cooking spray. Be generous! Eggs tend to stick. Muffin liners are a good alternative to avoid egg sticking to your pan.
Cook the spinach: Heat up a skillet over a medium flame and add the spinach. No need to add oil. Add salt and pepper. Cook the spinach, stirring occasionally until it has defrosted and the water has cooked out - about 10 minutes on medium-low heat. The pan should look pretty dry. We don't want soggy eggs!
Meanwhile, whisk the eggs in a bowl. Add the milk and salt and pepper to taste (be generous). If using, add cheese. When the spinach is done, add the spinach to the bowl. Mix everything together.
Pour the mixture into the muffin tin - the cups will be almost completely full. They will expand a lot in the oven, but shrink back down when you take them out, so it's ok.
Place the muffin tin on a sheet pan and bake the egg cups for 20-25 minutes, until they're firm to the touch (but still a little squishy - not hard) and a little golden-brown on top. Let them cool completely before storing in the fridge. Enjoy them with English muffins, toast, bagels, wraps or on their own!