Preheat your oven to 350°F (175°C). This ensures that the muffins cook evenly and rise properly.

Grease a muffin tin with cooking spray or line with muffin liners to prevent sticking.

In a large mixing bowl, crack the eggs and add the milk, salt, and black pepper.

Whisk the mixture until fully combined and slightly frothy. This incorporates air, which helps the muffins to be fluffy.

In a separate bowl, combine the diced bell pepper, chopped spinach, diced red onion, and halved cherry tomatoes. This mixture adds flavor, color, and nutrition to the muffins.

Fold the vegetable mixture into the egg mixture gently to combine without breaking the eggs.

Pour the egg and vegetable mixture evenly into the prepared muffin tin, filling each cup about ¾ full.

Sprinkle the shredded cheddar cheese on top of each muffin cup. This will melt and create a delicious cheesy topping.

Bake in the preheated oven for 18-20 minutes, or until the muffins are set and a toothpick inserted in the center comes out clean.

Remove the muffin tin from the oven and let it cool for 5 minutes. This helps the muffins to firm up and makes them easier to remove.

Carefully run a knife around the edges of each muffin and gently lift them out of the tin.

Serve warm or allow to cool completely before storing in an airtight container in the refrigerator for up to 5 days.