Spray your muffin pan with some non-stick cooking spray.
Throw in Spinach if desired (The spinach sticks right to the cooking spray. No floating!)
Crack one egg into each well.
Sprinkle the eggs with a little salt & pepper or whatever seasonings that you'd like.
Bake in the oven at 350 for about 15 minutes.
You could cook them for a bit less if you wanted a runny yolk.

Storage:
Store your cooked eggs in a Tupperware container in the refrigerator for about a week.
Use your judgement though.
If they look/smell bad, I wouldn't eat them.
If you can't get to them in a week then freeze them.
I haven't tried this myself, but I heard it works just fine.
Reheat:
Toast your English Muffin or bread of choice
Microwave your eggs for about 30-45 seconds (microwave times vary)
Enjoy a tasty breakfast in less than a minute (maybe longer depending on how long it takes for the muffin to toast )

More Ideas And Options
Use Egg Beaters Egg Whites (low calorie/low fat option). One 16oz carton fills 10 out of 12 muffin cups
Add Ham slices
Add broken up cooked bacon to the top
Scrambled eggs work too!!
Add diced green pepper, onions, bacon bits, and shredded cheese (Just don't fill the cups too full if you want it for a sandwich.)
Replace the spinach with tiny broccoli pieces
Sprinkle some cheddar cheese on top while still hot
Put a little cream cheese on your English muffin