Pre-heat oven to 350 degrees. Grease a muffin tin well with cooking spray or oil, or use muffin liners.
Whisk together the eggs and milk. Mix in the oregano, salt and pepper.
Chop the tomato and bell pepper. Discard/compost the watery part of the tomatoes with the seeds. Crumble the feta into small pieces. Divide the tomato, pepper and feta (and quinoa if using) evenly in the muffin tin. Pour the egg mixture over the veggies and cheese.
Bake for 25-30 minutes, until lightly browned on the tops. Remove from oven and let cool for 10 minutes, then remove the egg muffins from the tin.
Store in the fridge for up to 4 days, or the freezer for up to 3 months.
Notes
If you don't use muffin liners, I recommend washing the pan right after removing the egg muffins. The egg can really stick to the pan.
Mixing in quinoa is a great way to add in a healthy grain, but I find it also makes the egg muffins a bit crumbly and messier to eat. I think it's worth the mess.
I didn't add in olives simply because I do not care for them. If you love olives, toss some in!
I recommend not using the watery part of the tomatoes (the inside part with the seeds). If used, they add a lot of extra liquid and can make the muffins too watery.