Preheat oven to 350 degrees.
Melt 1/2 tablespoon of unsalted butter in a small saucepan on top of the stove over low-medium heat.
Add chopped onions. Stir constantly until semi-translucent.
Turn heat off. Set onions aside.
Rinse frozen spinach in a colander. Place rinsed spinach in a clean kitchen towel. Twist the towel tightly to remove all water. Set spinach aside.
Crack six eggs and drop them into a large bowl.
Add shredded cheddar cheese, salt and coarse ground black pepper (or white pepper), prepared onions and prepared spinach (drained/thawed).
Whisk (or stir) together until spinach is smooth and mixture is well combined. Set aside.
Spray 6-cup muffin tin with vegetable oil cooking spray,
Use a 1-cup measuring cup to scoop mixture from bowl... and to pour in each muffin cup.
Try not to overflow the cups. If an overflow happens, scoop it up with a spoon and place in an empty cup or a cup that needs more mixture.
Bake 12 to 15 minutes. Muffins will rise dramatically while baking. They will deflate dramatically while cooling. Taste not affected.
Serve hot or warm.
FREEZER TIP: Place cooled muffins in a freezer bag. Seal tightly. Refrigerate or freeze until ready to use (up to 7 days). Warm in microwave for 30 seconds OR in a preheated 350 degree oven for 5 minutes.