Preheat the oven to 350°F (175°C).

Generously grease a standard 12-cup muffin pan with cooking spray or butter.

Finely chop the baby spinach.

Crack the eggs into a large mixing bowl and whisk until evenly combined.

Add optional salt and black pepper if desired.

Stir in the shredded cheddar and chopped spinach.

Divide the mixture between 10–12 prepared muffin cups, filling each approximately three-quarters full.

Stir the mixture occasionally while portioning to keep the cheese and spinach evenly distributed.

Bake for 15–18 minutes, until the centers are just set and the tops are slightly puffed.

Remove from the oven and cool in the pan for approximately 5 minutes.

Gently loosen the edges and remove the egg cups.

Serve warm or cool completely before refrigerating.