Preheat your oven to 400 F. Prepare a small rectangular baking dish with cooking spray.
Separate the egg whites and yolks into 2 bowls. It's hard to explain how to do this, so if you've never done it before, I recommend doing a quick youtube search for an egg separating method!
Add milk to the bowl with the egg whites and whisk. Pour the mixture into the prepared baking dish. Sprinkle 1/2 cup of feta and a good pinch of salt on the egg whites. Stir with a fork, then place in the oven on the middle rack for 20-30 minutes, until the egg whites are firm to the touch and slightly crispy around the edges.
Meanwhile, cook the spinach and tomatoes. Add the olive oil to a skillet over medium heat, followed by the chopped spinach and sun-dried tomatoes. Add salt and pepper to taste and cook over medium heat until the spinach has wilted, about 5 minutes. Turn off the heat. Stir in the remaining feta. At this point, transfer the cooked veggies to a plate lined with paper towel. Allowing the towels to soak up extra liquid will help avoid a soggy breakfast wrap!
When the egg is done, let it cool completely. Then cut it into 3 rectangles.
Assemble: Place 1/3 of the cooked spinach in the center of the wrap and try to shape it into a rectangle. Place the egg rectangle on top. Fold the wrap like an envelope: fold the short sides in first over the egg, then the longer sides. Place them on a plate seam side down.
Seal the wraps: Heat up a skillet and spray with cooking spray. Place a wrap seam-side down and let it cook for 3- 5 minutes over medium heat, until it's golden and crispy. Flip and cook the other side. Repeat with the other two wraps.
Let them cool completely before wrapping in foil or placing in containers. You can store them in the fridge for 4-5 days or in the freezer for 6 months! Make sure to remove the foil before reheating in the microwave.