Preheat your oven to 180°C (350°F). Lightly grease a 12-cup muffin tin with olive oil or use non-stick muffin liners.

In a large bowl, whisk together the 6 large eggs and 60ml (1/4 cup) of milk until well combined and slightly frothy. Season with 1/4 tsp salt and 1/8 tsp black pepper.

Add the finely diced 1/2 medium red bell pepper (about 75g), 50g (2 cups) of finely chopped fresh spinach, and 1/4 small finely diced onion (about 30g) to the egg mixture. Stir in 1 tbsp chopped fresh chives and 50g (1/2 cup) shredded cheddar cheese, if using.

Gently stir all the ingredients together until evenly distributed. Be careful not to overmix.

Evenly divide the egg mixture among the 12 prepared muffin cups, filling each about two-thirds to three-quarters full. Ensure the vegetables and cheese are distributed well.

Bake in the preheated oven for 18-22 minutes, or until the egg muffins are set and slightly puffed. A toothpick inserted into the center should come out clean.

Let the egg muffins cool in the muffin tin for a few minutes before carefully removing them. They can be served warm or at room temperature.