1.) Heat oven to 350 degrees F.
2.) Heat the olive oil in a cast iron or oven-safe skillet over medium-high heat. Once the oil is heated, add the diced onions and kale. Season with salt and pepper, then allow to cook until kale reduces in volume and onions become translucent (about 5 minutes).
3.) In the meantime, crack the eggs into a mixing bowl and add the milk. Gently whisk until combined.
4.) Add the sun-dried tomatoes to the kale mixture and stir to combine. Spread evenly in the skillet, then pour the prepared egg mixture over the top. Gently shake the skillet to evenly distribute the egg mixture. Top with additional sun-dried tomatoes (optional) and cheese. Do not stir.
5.) Allow to cook on the stovetop for 2-3 minutes or until edges are set. Transfer the skillet to the middle rack of your preheated oven and allow to cook 12-15 minutes more or until the center is set and edges begin to pull away from the sides of the skillet.
6.) Allow to cool slightly before slicing into six even wedges and serving. If storing leftovers, allow to cool before sealing and chilling in the refrigerator.